5 bucks says the Pinion Nut is loose. Either that or you have a broken ring gear. I was coming back from the Con and GENO was in front of me and I could hear it while we were driving, stopped, tightend it. Got back to Chico, he undid the third and found out that the ring gear had some broken teeth off of it.
I would agree with this, I had this happen to me a long time ago. When you are under acceleration the load will force the pinion up and off the carrier, when you are decelerating it will be driven into the carrier creating a low growling noise and a vibration.
