Sure, there is better stuff that will show lower wear over time compared to the the ST stuff, but it's not like it's destroying your differential.
As far as the LSD additive...I would add one bottle of the Ford stuff and see how it works and do a bunch of tight "figure eights" in a parking lot. If you still have the moaning/groaning after that, then add 2oz at a time until problem is solved.
Hammer