There are three ABS sensors, one each front wheel and one mounted in the rear differential to support both rear wheels. Ford has a problem with the rear sensorand the frequency of failure, high probability this is your problem, and the part costs around $28. If do do a search on this topic you'll see this is a problem. It's easy to replace yourself, the sensor is located on the front of the differential slightly to the right (pasenger) side top, it has a wire connector and is held in by one ten (10) millimeter bolt. It's easy, but a little akward to reach. Good luck.