on the sierra the centre diff in the box is a viscous as well as the rear, the front is open because if you have spinning rears the power goes to the front under load, when you are going round corners the car is designed to oversteer as you have more torque going to the rears and the centre diff is the weakest link

you would be better to swap the centre diff for a higher rated torque rating before shelling out on a lsd for the front as this is the weak point

from what i can remember, cts did various stages of uprated centre diffs