There is so much confusion on this. I thought the new thinking was that if the differential was a limited slip, which if I understand right is only used in the car models, then yes use the limited slip additive. If the rear end is in a truck or suv then the differential is the locking differential which does not need the additive. I know that the new synthetic grape smelling GM fluid said not to use the additive with a locking rear end in a truck.
I just use royal purple 75w-90 which has an additive and I have never had an issue with my locking rear end. I know the gov-loc is not really a "locking" differential, buts thats how I was told GM classified it in terms of using the additive.