This is for the 8.5 10 bolt. Remove the cross shaft retaining bolt, cross shaft and c clips and remove the axles. Before you remove anything else from inside the housing you must use a dial indicator to check the backlash on the ring gear. If you don't know what the backlash is then you will not know what to set it to when you install the posi. To check the backlash you must wrap something around the yoke and housing to hold the pinion perfectly still. If it moves ANY you will get an inaccurate reading. Set the indicator up so that it measures rotation of the ring gear. What you will be measuring is the space between the ring gear teeth and the pinion teeth. This is the backlash. This should be around .008 to .012 inch if the gears are pretty good.
You need a dial indicator with a magnetic base. Set the dial indicator up so that the plunger is perpendicular to the center line of the ring gear. The plunger will look like this line _ as related to the ring gear, which is the O _O. Changing the shims about .003 will usually change the backlash about .001.