What you have is a sensor that has either worked its way out slightly or has simply gone bad. These are easy to replace. First, try tapping the sensors back into their holes with a rubber mallet. If that doesnt make a difference, take it to a dealer and have them read the codes for you. Up to 95 were capable of diagnosing ABS codes without a computer. Once you find out which sensor is giving you grief, order it on-line. Sensors are about $200-$260 on-line. The good news is, they take about 10 minutes to change. Good luck.