I believe the "solenoid" behind the front bumber is actually an air pump. There are 2 wires going to it, one 12V to engage the motor, the other is a pressue sensor wire. You'll also notice a hose going to the diff and another going up along the passenger side of the radiator, terminating at the top. This is the air inlet for the pump and its up high to prevent water/mud from entering it.
Not sure what year you have, but for a 2002, these are the control voltages you should be seeing:
