when you run the engine hold the wheels does the spur gear still turn if it does not then it is either stripped or you need to tighten it. If it spins then you either have a stripped gear in your tranny or you didn't tighten the diff enough. That little screw in the diff is a pain in the @$$ but the best way i found to do things was to put the allen wrench in to keep the screw from turning then rotate the opposite tire towards the front of the truck. other than that i do not know what to tell you. you may wanna check the diff gear itself when i got my truck i recieved it used and i ran into the same prob. the diff gear was so old the inner piece that holds the ball bearings had completely melted