I have a 1998 GMC Jimmy. I have the 3 switch button. When I attempt to put it in 4LO, the light flashes and then kicks back to 4HI. On occassion this winter, it would kick out of 4HI back into 2HI. I had the mechanic where I work check it out. He stated that the vacuum hoses on the actuator were good. He was thinking it was probably the encoder motor. Is there a way to tell for sure? How can I rule out the 4x4 module(?) or the switch itself before replacing the encoder motor.