Well, sounds like you have some kind of drivetrain issue, you might check your differentials and see if they're functioning normally.

If you put the back wheels on the ground and hold up the front wheels, when you spin one of the front wheels does the other wheel spin equally in the opposite direction, same goes for the rear end?

Also if you spin both wheels, does the drive shaft going to the center diff spin normally? If all of that seems ok, then I would suspect maybe the center diff or something in the tranny or possibly the motor itself.. When you try and go does it bog down like it's jammed, or just rev and nothing happens?

I think I've read somewhere that if you don't stop before you switch from forward/reverse you can snap the shifter shaft in the tranny, that might be it but you said everything inside it looked ok.

So yeah check all that good stuff and post back with whatever more info you can, thanks. [8D]