More than likely you have a wheel speed sensor that is going bad. The dealer should be able to hook it up to thier computers and tell you which one it is. Without knowing the specific one, you'd be playing a guessing game figuring it out (or replacing them all).
I had one go out on me about a year and a half ago. Same symptoms you describe. Changing the sensor is actually quite easy. Figuring out which one is bad is the tricky part.
