Im almost in the same boat mate. Had the box apart and everything is intact and runs as it should while turning by hand. Once the gearbox is fitted to the truck, give it some wellie and it just sits there and revs its nuts off. Wouldnt mind following this topic to see if anyone has a solution.
Also, both diffs in perfect order, all drive cups in order etc etc. No visible faults as far as I can see.