Squealing from rear end?

Today while driving I noticed a squealing noise coming from the rear end. I think I hadn't heard it until now because I recently took the bins out that came behind the rear seats, and I usually drive with the seats up

I work a lot, so it's not very easy to take to the dealer. Especially if I take it in and they can't find a problem.

Seems to do it at 15 mph and up

1. The squeal sounds almost like brakes are dragging, but it doesn't change when I hit the brake.
2. I would think the diff fluid would have to be awfully low to make that kind of squealing
3. I think U-joints usually make a more of a howling noise, almost like tire tread noise
4. Could it be part of the evap, or EGR system? It could be coming from gas tank area

Any thoughts, tips, and ideas appreciated

 

It could be a rear drum partially locked up.

 

Rear sliding window open a...CH?