I used a wider bearing on one side to take up the slop.. another guy at the track put two bearings in one side and that seems to do the trick too.. also I only put the gear diff in the rear.. not the front.. I like to have the brakes a spool gives.. if ur running a gear in the front.. I would put some really thick oil in there.. like 100K-300K
A couple of pages back u put a pick of ur gear diff in the car.. there is not that much slop.. only a couple of mm's..some shimms would work ok.. just not wide ones.. thin like the hot bodies diff shims..