F350 rear end locked up and Chirping
 

Ok. My husband has a 91 F350 with about a 6 in lift. The other day he came home and the rear end seemed to lock up on it. He put it in 4-wheel drive, took out the drive shaft and took all the gears out of the rear end (upon inspection everything "looks" good in the gears) so we could move it and it started making this awful, really loud "chirping" noise from somewhere near the rear end. Odd thing is it doesn't seem to make the chirping noise while he is driving in reverse, only moving forward. Any suggestions as to what might be the problem? Could a wheel bearing be the culprit? Thanks.