I have a 99 Yukon Denali and all the lights on the 4wd selector switch are on. The truck is neutral and ready to tow. When I try to select 2hi 4hi 4lo or auto 4wd. It won't select any of the above. Is it possibly the transfer case module?
The actuator on the transfer case if my first guess. Check to see if it is getting power to switch between selections. If there is no power, check relay. Also, if there is power, remove the actuator motor and check for operation.