The problem sounds like it's definitely in the diffs or the tranny. It's not hard to pull the tranny from the chassis. But going inside the tranny for the first time requires a little work. First with every thing the way it is. The black gear on the outside(AKA: Spur gear) spin the spur gear in the direction the engine would spin it. lift the truck off the ground. try to hear real closely as you spin the spur gear where the grinding noise is coming from. It might be a diff too. after that, yank out whatever part sounds like is grinding and fix it. heheheh, let us know what you come up with.
