The manufacturer recommended that there be one thousands of an inch clearence between rear shoes and the drum. Drum must be machined smooth, no just putting the shoes in and adjusting the breaks to size. A trick we used to do was to put some ajax in the drum to promote polishing, also shoes need to be name brand. Chevy has had a problem with this for years and sometimes is worse when it rains or the car is cold. I suspect the problem is the rear giving the sensation of one side grabbing also the fix has to be done bi-laterally. Now if new shoes were installed and ran the shoes would also have to be sanded or smoothed out. Good luck, I used to hate to see a chevy come in with this problem, hard to get costomer satisfaction, especially after discount repair stuff was used. Occasionally on the tough ones to fix we would buy from GM new shoes and drums. I am also assuming that you have bled the brakes and fluid is comming out of the bleeders at about the same rate and that if you have anti lock breaks that you have been servicing the system correctly.