I have also had this same problem except that I was getting damage to both front and rear diffs. I was rebuilding diffs every week. The manual states to put 2 shims on one side and four on the other. I have added an extra two shims on each side. So far the front diff has behaved after 3-4 races. Although the rear has improved I think it still needs more shims. Unfortunately its a trial and error process, you may need to sactrifice a couple more bevel diff gears before you find your right fit.