Rear brake noise

The rear brakes on our 2004Chevrolet Silverado (54000) make a strange winding sound when coming to a full stop. At just under 5mph is makes this noise. The brakes have been fully done over. Earlier this year while driving the truck, it felt like it was in 4 wheel drive and the ‘service 4wd’ light came on. I stopped driving it, brought to our mechanic and this is when he said it was the rear brakes… They feel ok otherwise. UGH!!