What is wrong when the back brakes on a 2003 Hyundai Elantra make a grating sound and the back tires lock up?
You have worn completely through the brake lining and the grating that you hear is the metal backing against the drum. They are locking up for the same reason and because the calipers are pushed out too far and are locking out. This happens a lot when people drive with their parking brake on over and over till the pads are gone. You have some expensive repair ahead which could have been avoided with an annual brake check, or just noticing when your car is behaving differently and checking it out, it will always give you signs of trouble before it gets as bad as it is now.