1994 G20 rear brakes grabbing HELP!!

I have a Chevrolet G20 custom van that has a rear brake issue. When stopping at low speeds, the rear brakes grab first, especially in gravel. I pulled the rear wheels and drums, expecting to find a leaking wheel cylinder or axle seal, because I have seen oil / fluid soaked shoes cause this problem, but no, they are dry, and look great.
Any ideas what would cause this? Master cylinder? proportioning valve? ABS unit? Global warming?? I need to get this fixed quick. Any ideas are GREATLY appreciated!! THANKS

Our 87 G20 with drum brakes on the rear was doing the same thing, getting slightly worse over the course of several thousand miles. I had determined the handbrake cables were stiff and not releasing fully. I think the slight rotation of the brake shoes each time I applied the regular brakes would have the effect of making the not-fully-released emergency brake be applied slightly by the actuator arm. I replaced the 3 handbrake cables (left, right, and front), and the slight grabbing problem was gone. I also cleaned and lubed the self-adjuster mechanisms, as they were not 'advancing', due to old grease and crud on the rotating screw of the self-adjuster.
I had tied a strong cord to the old front handbrake cable before pulling it out, which helped me snake the new cable back in. It saved me much frustration.
I had originally thought the problem was a bad wheel bearing, since the bearing on one side had some 'play' in it - maybe .030". In order to change the wheel bearings you have to remove the axle shafts. In order to remove the axle shafts you have to remove the pinion pin from the differential; and in order to remove the pinion pin you have to be able to unscrew the lateral locking bolt for that pin from the differential carrier. In my differential the hex head of that locking pin was snapped off (which turns out to be a well known GM situation), and I didnt have much time to mess with it. I will buy the GM diff. pin lockpin extractor kit in order to remove it someday.