If you run the slipper clutch check for this problem... With the pinon off motor and slipper set as normal , Spin the rear wheels and feel how free they spin. Now back off the slipper spring nut all the way and remove the spring, Does the Trans now spin even easier ? If so you need to shim the brake hex or get the new oversized one from the boy's in Detroit. There should be NO shims at all in trans top shaft inside the case and used only at brake hex/ motor plate interface. Shim the topshaft so it is in middle of its end play when at rest up on gear plate bearing. The slipper spring pulls VERY hard on the top shaft and if you have the end play not centered right the spring will place a ton of side load on top shaft bearing in trans case and its life will end in a very short time. Take your time and shim this area right so the endplay is centered and when the spring is cranked down so there is no side load inside trans case. All the pull load should be on the brake hex and gear plate bearing. The slipper should free spin with its spring on or off...........