Q: My 4WD is not engaging. Is a 2007 Silverado have electronically controlled axle actuator or vacuum? Or is my problem in the hubs?

Hi there. The actuator on your 4WD system to your vehicle is electronic. Most of the Silverado 2500 model Chevrolet trucks have issues with the actuator failing in the front axle due to weather and road conditions. Check the harness to the actuator and make sure that it is connected to the actuator. If a tree limb was ran over, its possible to rip the wiring out of the actuator. If the wires are intact to the actuator, the I recommend replacing the actuator before replacing the hubs. If you have a grinding noise in the front by the wheels, then the locked hubs have worn teeth and needs replaced. If you need further assistance with your 4WD not working, then seek out a professional, such as one from Your Mechanic, to help you.