'89 Rear Brakes Grabbing Hard

[PeteA216]

Hello everyone,
I've got an '89 GMC Sierra 2500 4x4. It has rear ABS, which seem to be working, however I do have a problem that just started recently. When I hit the brakes, even very gently, the rears both grab violently often to the point of locking up. If I get them to lock up enough to trigger the ABS then driving after that, the problem is resolved. At first I thought it was just rust on the drums that had to be worn off, but it does it way too often and consistantly (If the truck sits for five minutes or five hours it still does it.) Oh, and it seems to follow the pattern, the colder it is out, the worse it is. Any help would really be appreciated since it gets really annoying in stop and go traffic especially with the blizzard we're experiencing right now.

P.S. I already replaced all components of the back brakes this past spring, new springs, shoes, brake cylinders and even new brake lines on the axle.

[maxwedge]

Could be heavy dust deposits, but I have also seen the proportioning valve do this.