From my experience of ALL Tamiya diffs, they don't need to be tight not to slip, if you have over tightened the diff, then you have stressed the screw, and it will just keep slipping.
Rebuild the diff with a new screw and nut, out of the car, and just keeping nipping up 1/8th of a turn till it stops slipping, and then tighten for feel, if you are wrenching down, you will damage the threads/screw///
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