With the wheels off the ground, what does it do if you manually spin the spur gear? Do the drive shafts spin with it? If not, check your set screws on the drive cups. Then, alsowith the wheels off the ground, try to spin both rears or both fronts at the same time and see if it spins the main drive shafts.. If a diff is bad it'll be locked up or just wont turn the drive shafts when ya turn both tires at the same time.. But if one diff went out, the other should still giveya 2 wheel drive..Good luck!