Well, I went back and re-read the thread and I thought the problem was your rear brakes. In the event that you didn't check the rear diff here's some ideas. I'm thinking it's the rear diff.
Take off the driveshaft and try turning a wheel. Is it hard to do?
Also, check the backlash in the diff by wiggling the input yoke. I bet you that there's more than a quarter inch rotation before the rear wheels start to move.
Check and see if the pinion nut it loose. It's most likely the culprit. It's located inside the diff's input yoke.
Zeke
