I don't think it's the power of the RB S7 breaking the diffs. While it's a potent engine, there are more powerful ones out there (RB C5, Rossi Black Magic,...) that aren't chewing up ofna diffs.

Make sure there is no debris in the differentials and the action is smooth (no tight points then turning the drive cups in opposite directions). Also, feel for backlash (it's the enemy). You may need to shim the conical gears inside to remove internal play (shim between the conical gears and the diff case).

Good luck