Why a suregrip?

Chrysler, was using the Trac loc (2 pinion gears) back in the 70s clear up until the present in pickups and Jeeps. In both the 8 1/4, the 9 inch and the Dana 60 the 70. Same unit Ford used in their Dana's.
Although it's not as strong as the Power Loc(4 pinions) that was used in the 8 3/4, the Dana 60, 70 and 80 used by Chevy and Ford also.