Rear anti lock brake light stays on

1992 Toyota 4-runner 82,000 miles 6 cyl - the rear anti lock brake light stays on, and one service station told me it was probably a sensor and that I would probably have to pump my brakes. This has been fine until recently on a wet hill I had to come to a stop and found that I could not do so even pumping as normal. As I applied more pressure the rear end started to swing to the left. I have front brakes but the rear really does not seem to be there. I plan to have it really looked at seriously, however my question is if the sensor is what is wrong, could that cause the brakes to fail, or is it merely telling me that the brakes are not working. Some sensors are needed to tell the brakes what to do, others are just idiot lights telling YOU to have it serviced. I have heard horror stories that it could cost thousands to repair the rear anti lock brake system and my poor old SUV is not worth that much and I can't afford thousands for repairs nor to buy another vehicle.