I will be undertaking putting in a posi differential in my Grand Marquis. I'm not doing anything fancy, just want to swap out the factory peg leg for a factory posi (keeping the original ring and pinion). The procedure looks straight forward enough, but I have a question about the backlash. Since the differential only will change, do I need to check the backlash and contact pattern? Both units are factory parts - could I get away with just putting back the shims that were there?