Check underneath the battery tray for the diaphragm that actuates the front diff. If that diaphragm isn't working, then either it is shot, or you have a vacuum leak. That, or the switch on top of the transfer case is bad. The latter is unlikely though. The most often problem with the Chevy 4x4 system (if it is stock) are the vacuum lines or the diaphragm actuator. You can replace the actuator with a pull cable, though I haven't succeed in doing that yet,