with reference to fig1 of the drawings , a vehicle having a propshaft assembly that is constructed in accordance with the teachings of the present invention is generally indicated by reference numeral 10 . the vehicle 10 includes a driveline 12 , which is drivable via a connection to a power train 14 . the power train 14 includes an engine 16 and a transmission 18 . the driveline 12 includes a propshaft assembly 20 , a rear axle 22 and a plurality of wheels 24 . the engine 16 is mounted in an in - line or longitudinal orientation along the axis of the vehicle 10 and its output is selectively coupled via a conventional clutch to the input of the transmission 18 to transmit rotary power ( i . e ., drive torque ) therebetween . the input of the transmission 18 is commonly aligned with the output of the engine 16 for rotation about a rotary axis . the transmission 18 also includes an output 18 a and a gear reduction unit . the gear reduction unit is operable for coupling the transmission input to the transmission output at a predetermined gear speed ratio . the propshaft assembly 20 is coupled for rotation with the output 18 a of the transmission 18 . drive torque is transmitted through the propshaft assembly 20 to the rear axle 22 where it is selectively apportioned in a predetermined manner to the left and right rear wheels 24 a and 24 b , respectively . with additional reference to fig2 and 3 , the rear axle 22 is shown to include a differential assembly 30 , a left axle shaft assembly 32 , and a right axle shaft assembly 34 . the differential assembly 30 includes a housing 40 , a differential unit 42 and an input shaft assembly 44 . the housing 40 supports the differential unit 42 for rotation about a first axis 46 and further supports the input shaft assembly 44 for rotation about a second axis 48 that is perpendicular to the first axis 46 . the housing 40 is initially formed in a suitable casting or stamping process and thereafter machined as required . the housing includes a wall member 50 that defines a central cavity 52 having a left axle aperture 54 , a right axle aperture 56 , and an input shaft aperture 58 . the differential unit 42 is disposed within the central cavity 52 of the housing 40 and includes a case 70 , a ring gear 72 that is fixed for rotation with the case 70 , and a gearset 74 that is disposed within the case 70 . the gearset 74 includes first and second side gears 82 and 86 and a plurality of differential pinions 88 , which are rotatably supported on pinion shafts 90 that are mounted to the case 70 . the case 70 includes a pair of trunnions 92 and 96 and a gear cavity 98 . a pair of bearing assemblies 102 and 106 are shown to support the trunnions 92 and 96 , respectively , for rotation about the first axis 46 . the left and right axle assemblies 32 and 34 extend through the left and right axle apertures 54 and 56 , respectively , where they are coupled for rotation about the first axis 46 with the first and second side gears 82 and 86 , respectively . the case 70 is operable for supporting the plurality of differential pinions 88 for rotation within the gear cavity 98 about one or more axes that are perpendicular to the first axis 46 . the first and second side gears 82 and 86 each include a plurality of teeth 108 which meshingly engage teeth 110 that are formed on the differential pinions 88 . the input shaft assembly 44 extends through the input shaft aperture 58 where it is supported in the housing 40 for rotation about the second axis 48 . the input shaft assembly 44 includes an input shaft 120 , a pinion gear 122 having a plurality of pinion teeth 124 that meshingly engage the teeth 126 that are formed on the ring gear 72 , and a pair of bearing assemblies 128 and 130 which cooperate with the housing 40 to rotatably support the input shaft 120 . the input shaft assembly 44 is coupled for rotation with the propshaft assembly 20 and is operable for transmitting drive torque to the differential unit 42 . the left and right axle shaft assemblies 32 and 34 include an axle tube 150 that is fixed to the associated axle aperture 54 and 56 , respectively , and an axle half - shaft 152 that is supported for rotation in the axle tube 150 about the first axis 46 . each of the axle half - shafts 152 includes an externally splined portion 154 that meshingly engages a mating internally splined portion ( not specifically shown ) that is formed into the first and second side gears 82 and 86 , respectively . fig4 depicts the propshaft assembly 20 to include a shaft structure 200 , first and second trunnion caps 202 a and 202 b , first and second spiders 206 a and 206 b , a yoke assembly 208 and a yoke flange 210 . the first and second trunnion caps 202 a and 202 b , the first and second spider 206 a and 206 b , the yoke assembly 208 and the yoke flange 210 are conventional in their construction and operation and as such , need not be discussed in detail . briefly , the first and second trunnion caps 202 a and 202 b are fixedly coupled to the opposite ends of the shaft structure 200 , typically via a weld . each of the first and second spiders 206 a and 206 b is coupled to an associated one of the first and second trunnion caps 202 a and 202 b and to an associated one of the yoke assembly 208 and the yoke flange 210 . the yoke assembly 208 , first spider 206 a , and first trunnion cap 202 a collectively form a first universal joint 212 , while the yoke flange 210 , second spider 206 b and second trunnion cap 202 b collectively form a second universal joint 214 . the shaft structure 200 is illustrated to be generally cylindrical , having a hollow central cavity 220 and a longitudinal axis 222 . the shaft structure 200 is preferably formed from a welded seamless material , such as aluminum ( e . g ., 6061 - t6 conforming to astm b - 210 ) or steel . fig5 shows an insert member 250 may be inserted into the shaft structure 200 to attenuate shell mode vibration that is produced during transmission of rotary power by the propshaft assembly 20 . in the particular example provided , a single insert member 250 is employed . the insert member 250 is a substantially cylindrical structure having a shape that is generally complimentary to the inside surface of the shaft structure 200 . in the embodiment illustrated , the insert member 250 is configured as an elongated cylinder with a generally circular cross - section . the insert member 250 is further defined by a plurality of closed cells 252 interconnected to one another . in the exemplary insert member 250 , closed cells 252 are arranged in a honeycomb pattern where each cell 252 includes a substantially hexagonal cross - section . each cell may be shaped as a right hexagonal prism having a predetermined length . a cell length “ l ” ranging from about 1 mm to 2 mm is contemplated to provide desirable stiffness and energy absorption characteristics for at least one shaft structure having a known length , diameter , wall thickness and material . because the frequencies at which the shell modes are excited vary from component to component , the length and width of the cells may be varied to tune the insert to isolate certain frequencies . in similar fashion , the insert material may be changed to target certain frequencies for attenuation . one embodiment of the invention utilizes an insert member constructed from polypropylene . other materials such as aluminum may also be used . insert 250 is constructed from a material having a compressive strength in the range of 140 - 250 psi . this compressive strength is sufficient to resist the radially inward deflection of portions of the shaft structure . as such , the insert 250 increases structural stiffness of the tube to provide energy absorption during the shell vibration modes . the insert 250 includes an outer surface 254 defining a first outer diameter when insert 250 is in a “ free ” or unloaded condition . the first outer diameter is greater than an inner diameter defined by an inner surface 256 of shaft structure 200 . to assemble propshaft assembly 20 , an adhesive 258 is applied to outer surface 254 . a force is applied to insert 250 to reduce the first outer diameter to a second outer diameter less than the inner diameter of inner surface 256 . insert 250 is positioned within cavity 220 where the external force is released . insert 250 is constructed from a substantially elastomeric material such that insert 250 tends to spring back to its original un - deformed stated . shaft structure 200 resists this tendency and an equilibrium is reached where insert 250 biasedly engages shaft structure 200 . the biased engagement as well as the adhesive bond between the insert 250 and inner surface 256 assures that insert 250 maintains a proper load - transfer - type engagement with shaft structure 300 . fig6 - 8 depict a first shell mode of vibration of shaft structure 200 . fig9 - 11 depict shaft structure 200 in a deformed state while in a second shell mode . in the first shell mode depicted in fig6 - 8 , portions 260 of shaft structure 200 move radially inwardly towards longitudinal axis 222 while portions 262 move radially outwardly from longitudinal axis 222 . the maximum amplitude of deflection during a first shell mode occurs at approximately the midpoint along the length of shaft structure 200 . the maximum deflection location is termed an anti - node . fig9 - 11 depict the second shell mode of vibration for shaft structure 200 . the second shell mode includes a first anti - node 270 and a second anti - node 272 spaced apart from one another along the length of shaft structure 200 . portions 274 located at first anti - node 270 deflect radially inwardly while portions 278 deflect radially outwardly . the radially inwardly deflecting portions are substantially diametrically opposed from one another as are the radially outwardly deflecting portions . the radially inwardly extending portions 274 are aligned along an axis y while the radially outwardly extending portions 278 are aligned along an axis x orthogonal to axis y . radially inwardly extending portions 280 are substantially diametrically opposed from one another and axially located along shaft structure 200 at second anti - node 272 . the radially inwardly extending portions 280 are positioned along axis x . radially outwardly extending portions 282 are substantially diametrically opposed to one another and aligned along axis y . the magnitude of deflections both radially inwardly and radially outwardly at second anti - node 272 are substantially similar to the magnitude of deflections located at first anti - node 270 . however , the shell mode shape of second anti - node 272 has been rotated substantially 90 degrees about longitudinal axis 222 in relation to the shape of shaft structure 200 at first anti - node 270 . while the invention has been described in the specification and illustrated in the drawings with reference to a preferred embodiment , it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention as defined in the claims . in addition , many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof . it is , therefore , intended that the invention not be limited to the particular embodiments illustrated by the drawings and described in the specification as the best mode presently contemplated for carrying out this invention , but that the invention will include any embodiments falling within the foregoing description and the appended claims .