On my Pick U.P. and Suburban, both half tons, I removed the rotors to access the sensor on my the hub. There is an Allen head screw that holds the sensor into the hub. Remove the screw and pull the sensor out to clean any debris on the magnet. I put a small piece of rag in the hole and wire...
Take the sensor out of the hub and make sure that it is clean it off real good. I have done this on my Chevrolet trucks and it has stopped the ABS from kicking in at slow speeds. At least it will tell you if the sensor is good or bad.