Center diff and direct drive should have the same tooth count so that they mesh in the trans housing. If you're locking the truck in second and running the standard gear ratio, you'd pick second standard. If you're using the close or wide ratio, you'll have to choose appropriately, obviously. The only gear ratio changes available for the trans are the close and wide set, which replace the input cluster gear and one of the 2 speed gears. CD and FOC have the same ratio.