I too have noticed this. It seems to happen after a little bit longer drives (20 minutes +), and when it’s warmer out. At first I thought it was the brakes, but since most of the driving I do is highway, I don't think I would have enough heat built up in them to groan. I'm guessing it’s the diff, even though it really does sound like wheel or brake rub. I have a friend in Houston with a 06 GT who noticed the same thing, took it to the dealer, and they told him it was all in his head. It may be just me, but it sounds more prevalent when you make tight left turns.
<<EDIT>>
Look and ye shall find:
http://www.stangnet.com/images/stories/docs/s197_TSBs/06-19-02.pdf