There is a sensor in the left and right front wheel and one in the transfer case. The ones in the wheels, you can try to take them out and clean them and see if this helps the problem. Another suggestion is to have the vehicle scanned at your local parts store (which they should do for free) to isolate which sensor is causing the problem. If the one in the transfer case is bad, it usually causes problems with the speedometer. More than likely it is one of the sensors on the wheel. Make sure to also check the wiring going to the wheel speed sensors to make sure that they are not broken. Hope this answers your question; thanks so much!!