ahhh.... the infamous RTR syndrome.
remove the rear wheels. remove the cvd axels. inside the hubs that attach the rear diff u will see a small allen screw on one side. insert an allen wrench, and turn that screw nice n tight (but no too tight) while holdint the diff to prevent it from spinning. if ur truck came with instructions on how to build it , look towards the beginning, and it will show u how to do this. if u still can't figure it out, lemmi know, as the RC10GT diff is almost the same, and i can try and show u some pics of it.