Come drive on our outdoor asfalt tracks then YoDog... You WILL need to brake in some turns...
If the diff is really okay, meaning it is build so that the spurgear just doesn't slip, there could be other problems...
You may have checked the car's tweak but did you check if the chassis is straight? Check that with a caliper body or a straight ruler of some kind. Try to loosen the screws of the front suspension and then gently tighten them again cross-wise.
Check for any binding in the centerlink, sidelinks, shocks, springs and tubes.
Make sure the kingpins are smooth without any binding.
Make sure the front springs are the same rate AND are equal in lenghth.
Check the tires, they should be the same diameter left and right.
Check the balance left & right of the complete car. If it is much out of balance, get it in balance by re-mounting electronics or add weight in places needed...