[Big-Foot]

Have you had any work done on the rear end? Differentials will act that way when the gears are having "interface" problems. When the lube is cold, the noise is much less or almost non-existant. As the lube warms up, the noise and vibration increase with intensity. The noise and vibration will come and go as you load or unload the differential also.

I would put her up in the air and see how much rotational backlash you have with the pinion.