check it buy turning your rear wheels. They should spin freely, if they dont want to turn your binding up some where. Slipper clutch I tighten down, but then again I dont much like wasted power =)~.  As for fixing your diff, well I cant offer any better advice then to visit Traxxas website or look in your owners manual for the exploded pics on how things go together. I am NOT a good teacher sorry =/