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It sounds like you may have a bearing failure in the rear differential. If it were a gear noise then it would be described more as a whine, where a failed bearing or race would give a growl type noise.
There are a few ways that bearings can fail.
Preload can be too high. For instance if the pinion seal had been replaced and the nut was over torqued it would cause pinion preload to be too high and the bearings will fail.
Fluid contamination can cause bearing failure. If there was water in the fluid from going through a large puddle or something like that then it will cause failure of the bearings and eventually gears also.
Failure can happen from fluid degradation and lack of period maintenance.
It can also just be a failure of the bearing or race that is unpreventable.