Differential question

I just found out, doing an all-fluids change myself, that the rear differential on my '04 Z71 Avalanche (4.10 locker) has been 2 quarts low since I've had it....which is 26 months now. No leaks or seeps anywhere on
the diff. Lest I be accused of lack of due diligence, I took it to the GM dealer a month after getting it an had all the fluids "serviced"...including the front and rear diffs and the transfer case. After spending a day under there I can tell with 100% certainty that none of the drain plugs on the t/c or front diff have ever been off. They are as pristine as they were when they left GM's Mexico plant. Ditto for the rear diff plate and fill plug. All along I though a certain amount of diff howl was normal with the 4.10's. Now I wonder if it was low fluid all along. Question: One What's the capacity of this differential....how much fluid has been in there with 2 quarts missing? Question Two: What's the likelihood of permament damage to the rear end? I wiggled the rear wheels while off the ground and the wheel bearings seem tight. No real noise or play apparent.What are the odds this has damaged the diff?
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Update: Upon checking it was about 1.75 qts low, not quite 2.
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