The 4S is the issue. He is running a LOT of power.
Robinson is the only one I know that made an alum diff gear, those may have only been for the gas truck.
You should be able to get the diff on the tight side and the slipper so it does not give a lot but will save the diff. Just make sure the slipper gives before the diff does.
To do this run this check.
Tires on the car gear cover off:
Hold the right tire and the spur gear in your right hand and then turn the left tire while looking at the nut on the top shaft. When the left tire is rotated you should be able to see the top shaft nut rotate. If it does the slipper is giving before the diff. If the nut does not move the diff gave before the slipper and you need to tighten the diff or loosen the slipper.
The stock parts should take a good beating if set properly but a 4S is pretty insane.
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