I'll be shopping for a new (or new-ish) truck shortly for pulling a medium size trailer or 5th wheel. It seems like the vast majority of 3/4 and 1 ton trucks around here are 4WD, with jut a few 2WD...
Get a rwd with locker, and You will get out of most situations. If you need 4lo, then you do not have a low enough gear in the axel, trans or some combo there of.