The diff shouldnt slip at all - thats the job of the slipper clutch.
Hold the left back wheel with your left hand, use the thumb of your left hand to jam the spur gear, now try and turn the right rear wheel with your right hand. It should be almost impossible to turn using moderate force. If its slipping easily then your diff needs tightening a little.
Back the slipper right off and run your car on some grass or other high grip surface. the slipper will be slipping like mad at this point. Now slowly tighten it a bit at a time until there is just the smallest amount of slip before the car accelerates forward. This will protect your gears when landing from jumps etc and is the correct setting for the slipper.
HTH
