4WD will not work. the front differential will not engage but the transfer case does.
Year, make, and model please. There are a lot of different 4WD Chevrolets out there.
The actuator is located under the battery tray on my 2000 Blazer 4wd with Autotrac. Vacuum to the actuator is controlled by a vacuum switch located in the center of the firewall. The switch can be bypassed by removing and connecting the hoses on each side. If the actuator is good the front differential should engage.
I’m not sure where the switch is located on the Controltrac system. Also check the vacuum hoses for leaks and deterioration.