May I suggest checking allelectronis for gear head/ geared motors?

They have all sorts of gearhead motors , stock changing regularly. If you run a gearhead to a transfer case, say some 1/4" flat stock aluminum, housing four bearings, two shafts and drilled out pinions, ... you got a transfer case needing some drive cups or drive shafts.

I've found brazing tubing is the easiest chassis if you have a torch, chopping aluminum stocks fun if you have a cut wheel....?