My rear end is about to explode!!!!

[93blkGTfun]

I am completely baffeled. Someone please help me, I have been working on this for three days, and am completely stumped. I apologize for this being so long. My mods that would be in the effected area include an off road h-pipe and a transmission by Jasper engines.

About two months ago I started to hear a slight rubbing noise under the back of the car. Over time it has gradually increased in frequency and in loudness. It is now to the point that it sounds like nails scraping down a chalk board that is louder than my exhaust at wot.

I thought I had a brake problem, so i checked the shoes and drums. Nothing out of the ordinary. I checked the u joint in the back of the drive shaft, no play no hard spots. So then I took the cover off the diff. The oil was a little blackened but there wasn't a single sliver of metal in it. I did notice that when i turn the back wheels with my hand, about one quarter of a full revolution is harder to turn, than the other 3 quarters.

The car only does this under acceleration if it is coasting, it does not do it. It does not do it in reverse. The kicker is if you jack it up on the rear end and run the car it does not do it at all! The h- pipe does not rub anywere. I am going to class and I'll be back in about 2 hours to answer any questions. My rear end really does sound like its about to explode.