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Rear axle end play adjustments and how it works.
back plate off

If you look at the center of the above picture the horizontal thrust block can be seen, it is between the side bevel gears and around the shaft that supports the differential pinion gears at the top and bottom of the differential case. The thrust block stops the axles at the correct distance into the side bevel gears so the bearings at the end of each axle will have the correct end play. The left side axle (drivers side) must  have zero clearance from the thrust block to the outside race on the bearing at the end of the axle (drivers side). The right side axle will have from .001 to .006 clearance that is controlled by shims between the end of the axle housing and the brake backing plate. The clearance is on the right side axle (passenger side) so any force placed on the ring gear differential case will try to force the ring gear to the right and not to the left. This is because the ring gear is set with .003 clearance to the left of the pinion and any more clearance would put excess pressure on the teeth of the ring and pinion gears.
diff exploded
This is the differential and rear axle exploded view. In the above paragraph items  20 (differential case), 16 (pinion), 22 (ring gear), 24 (side bevel gears), 26 (differential pinion), 27 (shaft), 31 (thrust block), 32 (axle)  were discussed.