I have 2 489 and one 741 case. All set up with power lock units. One of the 489’s has the doctor diff unit and the other 2 are o.e. Units. 2 are 3.23 and the other is 3.91 gear set. No complaints with any of them with the additive, although the doctor diff can be a bit ‘clunky’ on real tight slow left hand turns occasionally. I do think he sets his up fairly aggressive for harder hits. Again no complaints. Hope this helps. Swingn’