They are pretty difficult to get unseated as they are surrounded by rust. I would clean the area, remove the mounting bolt, and keep squirting penetrating oil around the mount for several days and then pull the tire off and then finally try to take the sensor off. Be careful not to pull the wires loose and be careful not to break the plastic housing when trying to remove it. The sensor itself might be dirty but I would bet the sensor is having trouble because of rust or dirt or something on the axle shaft so you might have to pull the shafts out to inspect and clean them. If you get to that point, you will see how the magnetic sensor reads wheel speed from blocky deviations on the axle shaft and why there may be a problem. As a precaution, keep the sensor from contacting metal while you are working on things. Good luck!