The transmission may be bound up. If you can (if you can't, don't worry), take apart the diffs and transmission and look for bent shafts or dead bearings. As far as a brand favorite goes, OFNA/Hobao is what I like but their stuff is hard to get. I assume you want a monster truck/truggy? If so, HPI's Savage line looks like it'd be nice.

If you can't rebuild it yourself, I could do it for you if you really needed me to.