#1 and cheapest replacement: Encoder motor sensor, aka encoder ring in the transfercase selector motor
#2 selector switch
#3 front diff actuator
#4 TCCM transfer case module
This is of course just throwing parts at the issue. It could even be something mechanical but usually the encoder ring position sensor if the light just flashes when trying to change modes or sometimes is engaged in a mode other than that selected when last used.
Have you guys tried replacing anything yet? Any competent GM mechanic will properly repair your vehicle without throwing parts at it, only charging you for that which was faulting. Keep us up to date, don't leave threads open-ended.
I had the "SERVICE 4WD" light come on last week which I diagnosed as water in the master transfer case wiring connection. All I did was a drive in a bad storm on my way to work & my 4WD became completely disabled until I found the problem.
