2000 Chevy Cavalier ABS light

My ABS light is on and sometimes it goes off. But for the most part its on all the time. When applying the brakes the rear driver side tire locks up, which then the car will start sliding to the right. This happens 90% of the time. Usually it stops locking up after coming to the first stop. Then theres the time when the rear passenger side tire will lock up causing both tires to slide across the road surface.