Service 4wd after driving a while.

[Wildvortex]

I start my 02 envoy and there are no service lights lit. I drive for 15-20 minutes in 2wd and suddenly the service 4wd light comes on. What would cause this?

Prior to the light coming on I can put the vehicle in all 4wd ranges and hear both motors running then the light stops flashing and becomes solid in the desired position.

Once the svc 4wd light comes on it no longer will shift into the proper gear.

I have a new TCCM and diffential actuator.