Catalyzes the isomerization of L-xylulose-5-phosphate to L-ribulose-5-phosphate (Potential). May be involved in the utilization of 2,3-diketo-L-gulonate. L-ribulose 5-phosphate = L-xylulose 5-phosphate Belongs to the L-ribulose-5-phosphate 3-epimerase family. Extended N-terminus.