Catalyzes the reversible epimerization of D-ribulose 5-phosphate to D-xylulose 5-phosphate. D-ribulose 5-phosphate = D-xylulose 5-phosphate Binds 1 divalent metal cation per subunit. Carbohydrate degradation. Belongs to the ribulose-phosphate 3-epimerase family.