I believe Royal Purple 75W140 synthetic gear lube already has friction modifier in it's formula. Or actually, it's called a "limited-slip additive." I think they're the same. I use Royal Purple 75W140 synthetic gear lube.
Right now, I'm still using factory fill Mercon V from Motorcraft, but will be switching to Amsoil full synthetic transmission fluid soon.
For the rear differential, I use Amsoil 75w-90 severe service differential fluid. It works great. I highly recommend Amsoil products due to my experience with them. I've heard great things about Redline and Royal Purple as well. Whatever rear differential fluid you get, be sure to get a 4oz. bottle of frction modifier/ slip lock additive to be on the safe side.