Catalyzes the NADPH dependent reduction of D-ribulose 5-phosphate to D-ribitol 5-phosphate. D-ribitol 5-phosphate + NADP(+) = D-ribulose 5-phosphate + H(+) + NADPH Cell wall biogenesis; poly(ribitol phosphate) teichoic acid biosynthesis. Heterodimer together with TarI. Belongs to the zinc-containing alcohol dehydrogenase family.