Well camper season is right around the corner so I finally got off my ass and email Bilstein for an objective answer on their shocks. Here's the gist of it.
4600: stock height, which we all knew, digressive valving just like the 5100. Supposedly valved for the spring, which I find difficult to believe, since the same front is spec'd for the 1500/2500/3500 and they all use different rate torsion bars. The rears are the same 1500 and 2500 which, again, stupid because between the 2 you can have anywhere from 3-5 main leaves and there are at least 3 different overloads. At least the 3500 gets its own shock out back.
5100 (0-2"): valved for a cranked torsion bar, but again the logic here seems about as sound as in the case of the 4600.
5100 (4-6") again, valved for the preload in this instance, so I ASSume identical to the 0-2" shock just longer.
In the end what I gathered is that they are not identically valved, but they are very similarly valved, so 4600 it is for the dually. Problem solved.
