Catalyzes the interconversion of L-ribulose 5-phosphate (LRu5P) and D-xylulose 5-phosphate (D-Xu5P) via a retroaldol/aldol mechanism (carbon-carbon bond cleavage analogous to a class II aldolase reaction). May be involved in the utilization of 2,3-diketo-L-gulonate. L-ribulose 5-phosphate = D-xylulose 5-phosphate Binds 1 zinc ion per subunit. Belongs to the aldolase class II family. AraD/FucA subfamily.