Grinding noise/ groan

Alright, I'm at my wits end trying to figure out what is causing a noise from the rear of the vehicle. So far I've replaced the rear wheel bearings, the rear u-joints, and I've been in the rear differential and there seem there is nothing wrong internally. The "groan" is only on very slow turns and sounds almost like tire rub or metal grinding at times.......any ideas? Car is a 2003 mustang gt 5spd with an 8.8in rear-end with 38,000 miles.

I just looked at it internally myself, when I did the bearing........only thing I can mention, I don't think I had LSD additive in with it, so I put some in this afternoon, didn't seem to help at first, but on my way home the grinding stopped for a bit, and I wonder if that was it..........too soon to tell, usually how much does a rear-end rebuild cost if you're having someone do it..........if its anything like having gears installed I may be better off buying a new one with 4.10s in it. Thanks for the help by the way, I really appreciate any input.

What is the easiest way to identify if the bushings are the problem?