I think it's squealing all the time even when I roll it manually, although in that case it's less noticeable I guess since I can't turn the wheels fast enough.
I've had my diff set pretty damn tight and it doesn't help reducing the noise. Whether it's set loose or tight I can't really tell a difference in the noise.
The slipper (I do have one) is set at a moderate level, pretty much at the same level as it's always been set since I got the car. I even put a double nut to avoid self tightening or loosening.

I made a small 20s mp3 in case someone experienced could tell from the exact noise what it could be:

[link]http://www.freewebtown.com/flyojumper/Diff.mp3[/link]

It's only 251KB.

You can tell the squealing easily at the end when I maintained a low speed.