the chevys had a problem with the 4wd actuator engauging(sp) in colder weather...or just craping out all together! They redesigned the solenoid and made it into a plastic houseing. If you look on the front diff assmbly from the front of the truck, there should be a can type looking thing to the left side mounted on top of the axle tube with two wires coming out of it. That most likely is the problem. You can go tothe local dealer and get the replacement piece. The one i got didnt come with instructions and had to get it faxed over from the parts dept. So make sure you heck this before you leave!! The new one come with a third wire that needs to be spliced into the wiring harness over by the transfercase. Its an easy job you can do in about 30min or less.