the pinion gear is installed through the 2 8X16 bearings that are installed in the diff case. then the center shaft is installed onto the pinion gear and the
set screw is tightened (with loctite on the threads). there should be little to no for/aft movment of the pinion gear at this point. you would then shim the diff gear so as to have no side to side play, and some gear back lash.
the manual is availible at mugen seiki for download.
HTH, Dave