Using shims of different sizes, they move the carrier with the ring gear to the right (tighter) or to the left (looser) to make up the space between the gear set. With an indicator on a tooth of the ring, it should rotate approx .0100" before the pinion starts to move.
Its happened to me before, thought i'd throw in some advice. If you've alleviated that it is not the rear end or a gear issue, then what i did is i checked the rear brakes. If you have rear disc brakes and not drums sometimes you can clamp the caliper onto the rotor a little to tightly, causing the pads to rub when they heat up and cool down quickly. I.E. before you get on the highway you stop at stop lights warming them up then get on the highway and they cool down and expand. Although thats if they're riding the rotor. Just a helpful hint and maybe one more alleviation to your issue. Hope it helps.
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