RepairPal Certified shops can diagnose and repair your 2001 GMC Savana 3500. They provide transparent pricing with a minimum 12-month or 12,000-mile warranty on repairs and services.
If the rear wheels locked up on you at the same time, my first thought is to have the differential inspected, not the brakes. You can rule the brakes out by having someone press them while you watch/listen and see if they are actuating.