START FROM THE BEGINNING

1. Put the rear of your Corvette securely on frame stands so both rear wheels are off the ground

2. Grab the tire with both hands perpendicular to the ground. With one hand at the top and one hand at the bottom apply a substantial amount of force. You will feel some movement. This movement is differential yoke endplay.

3. Grab the tire parallel to the ground with both hands, apply force very lightly, just enough to feel a little movement. This movement is rear spindle bearing endplay.

ENDPLAY

When I rebuild a differential it will go out the door with zero yoke endplay. I would like to think the yokes would stay at zero endplay, however, they don't.

The main cause of the differential yokes loosening up is the posi clutch pack. The side yoke is installed in such a way that it passes through the outer case, through the posi clutch pack and through the side gear. A snap ring holds the yoke in place.

The posi unit is made up of discs similar to a motorcycle clutch. The discs are made of metal and submerged in gear oil. As soon as a new posi clutch pack breaks in, because they are metal on metal, they will loosen up slightly. This causes endplay.

The second cause of a loose yoke is the yoke itself. From about 1978 to around 1981 G.M. had a problem with one of their outside venders. The vender did not properly heat treat the side yokes.

I replaced hundreds of 78 and 79 side yokes at 35,000 miles or less. Some yokes would be worn down as much as 1/4 of and inch to where the snap ring groove and snap ring were gone. Usually the snap ring went through the gears without causing too much damage. Sometimes the outer case seal and case would be destroyed.

The yoke would wear down so much that it would cut into the side of the differential case. I would guess that by now most 78 to 82 Corvettes with defective side yokes have probably been repaired.

HOW MUCH ENDPLAY

How much is too much endplay? I would say a Corvette differential, assuming the clutch pack is shimmed properly, should not have over about .050" endplay. Most Corvette's that come into my shop have between .060" and .125" endplay.
If the endplay is from a very loose clutch pack the differential might chatter when making a sharp turn. If the differential chatters add some G.M. posi additive and hope it works. 

WHEEL BEARING ENDPLAY

When I rebuild a rear torque arm assembly I try to set the endplay as close to .001" as possible, never over .002".When you check rear wheel bearing endplay, if the bearings are set up properly, you will barely be able to feel the movement. This is good. The factory allows up to .008" rear wheel bearing endplay. .008" is way too much endplay and the rear wheel will feel very loose. 

CONCLUSION

Check the yoke endplay, check the rear wheel bearing endplay, determine which is at fault and proceed ahead. If the yoke is very loose, pull the differential, shim the clutch pack, replace the the yoke and reassemble. Rebuild the complete differential if it makes you feel better. If the rear wheel bearings are loose, pull the rear torque arm assemblies and replace the bearings and shim them to .001".