I have the same issue. I don't know if it makes any difference, but I just got a new diff, and the same problem is still happening. I have 90k + miles on my car, and they said a tooth was out of the diff and needed to be replaced when I took it in for a flush. At least it was still under warranty.
I had the noise when I was stock with a hitch. The problem continued with AP Stage 1. I've kind of always thought it sounded like it came fom the exhaust, but I haven't spent much time on the problem.
'05 LGT
AT