Are you sure it's the slipper and not the diff?
Take the gear cover off, and hold the right rear tire and the spur gear with your right hand. Now try to turn the left tire. If the slipper spring and nut is turning, then it is the slipper. If they are not, it's the diff.
Tighten the slipper nut until the left tire is considerably difficult to turn using the same test. If it is the diff that is slipping, place an allen wrench through the slots in the outdrive when lined up with the slot in the diff screw, and turn the opposite tire clockwise (in very small increments). The slipper spring and nut should always turn when using the above-mentioned test.
Aaron Waldron - LiveRC