powertrax is for converting an open to a psuedo posi (clunks on turns) more geared for low speed off roader trail trucks
you cant use 30 spline axles in a stock 28spline carrier that came in our 8.5" regals , you would need an open carrier from a 88 or newer 8.5" which have 30 spline axles from the factory
i would seriously reconsider the powertrax , they dont like being disengaged (when a wheel slips ) and then reengaging at high speed and high torque that we make
if you want posi go eaton posi which has 400# clutch pack get the 30spline unit with the appropriate install kit with larger diameter bore bearings and races (also called spool bearings) pop out your old peg leg carrier and swap the ring gear over , pop in the new eaton posi and the 30 spline axles will slide right in , and you wont clunk on turns