Probably a bad pinion bearing...
Parts should be available most anywhere that sells bearings. Fix it before the misalignment damages the gears. Unfortunately it's alot of labor to have to pay for if you can't do it yourself. I'd recommend replacing all the bearings while you have it apart; that would be both pinion bearings, both axle bearings, and the carrier bearings.
Most of the time, this is caused by running it dry, or by having water in the fluid.
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