brakes locking up

I have a 1997 Sierra , last month the back wheels or wheel locked up tight . Had to have it towed to the dealer , they couldn't see anything that was wrong . They turned down the drums and it worked fine for a week or so then started again . Had the rear end disassembled and checked , could find nothing , althought it has not locked up since. However , if I hold on the brakes when stopped , I can feel a release when I begin to move. Also before when they did lock up , I could put in in reverse and they would free up . Some people suggested it could be the ABS system , but the dealer says if that was the case a light would be on . Any ideas that I can take but to them . Thanks Kevin / N.S.

I have a 1997 Sierra , last month the back wheels or wheel locked up tight . Had to have it towed to the dealer , they couldn't see anything that was wrong . They turned down the drums and it worked fine for a week or so then started again . Had the rear end disassembled and checked , could find nothing , althought it has not locked up since. However , if I hold on the brakes when stopped , I can feel a release when I begin to move. Also before when they did lock up , I could put in in reverse and they would free up . Some people suggested it could be the ABS system , but the dealer says if that was the case a light would be on . Any ideas that I can take but to them . Thanks Kevin / N.S.
Do you have a locking diff? When this happens you need to jack it up and try and determine if one wheel is locked. The diff can cause this and it will not be visible on inspection with the Eaton locking diff.

No I do not have a locking or Slip? diff . if that is what you mean . Kevin / N.S.

Here I am with an update over a year later. Problem persisted off and on and became worst last month , finally the barkes started dragging constantly. Took it back to the dealer one more time . The tech replaced the brake springs and guess what the problem has not recurred. the return spring if that what you call it was weak and allow the pads to shift and bind. Simple problem , big pain in the ass.