Most of the newer synthetic 75w-90 and 75w-110 and 75w-140 already have a limited slip additive in the fluid, I know valvoline synpower does in 75w-90 and 75w-140, same with Mobil 1 and amsoil and redline ect ect.
I'm running valvoline synpower 75w-90 in my 8 3/4, but it's a 741 case with open 3.55 gears.
I would have confidence in the valvoline synpower in a sure grip unit as well.