Mine is an 02 and I think I read it isn't vacuum driven anymore. I found a great website called chevytalk and have a couple of leads. Some say it is the transfer case control module(tccm) and on rockauto.com there is a video to replace it(simple) inside the passenger side cabin where your right foot would sit. They say check it for corrosion on the contacts and clean it and apply dielectric grease to the connectors. I was going to try that before having my mechanic take over, he said one of the acuators was sticking. When I hit the switch to shift to 4wd hi or lo you can hear the module but it won't shift in unless in nuetural. Be back in a bit but I'll check back from my phone. When I said I was going earlier I dozed off in the chair reading up on the replacement of the part.