Another option is the worm gear axles from a losi mini crawler. Won't have the native gear reduction the tamiyas would, but they are v strong and wont break. Perhaps you could even hook them together.. IDK.
Tamiya also makes a
worm gear box like the one posted above.
I think Linc is right on making it FWD. May help with the pulling as well. RWD would likely just backflip. A crawler type veh seems best suited for this challenge