Can't speak for the Bilsteins, but according to Joe in Tech Support at Tenneco (Monroe), when I spoke with him about it a few years ago, he told me that the valving in the Rancho RSXs is the same for both the 2wd and the 4wd models, with the only difference being in the body as I mentioned before. The 4x4 shocks will work fine on a 2wd truck, but not the other way around as the standard 2wd shock body will make contact with the halfshaft.
-Joe