AAM produced some axles, prior to Aug. '05 that had lapping compound left on the gearset. Usually, within 3-5K miles, the axle would be too loud to be drowned out by a Monsoon system.
Some happened with earlier mileage, some after 5K; depending on the amount of lapping compound.
As the gearset wears, it can cause some vibration problems.
AAM foots the bill for this repair, which is a quick swap of the axle. Easy diagnosis, pull the cover and see if there is sandy substance floating in the lube.
I would have thought most are replaced, but evidently there are a few out there that are still with low mileage or with small amounts of compound.