snorander,
I have a 2003 ZR2 S10, it does not have the "vacuum switch" either. My four wheel drive would not engage but the lights on the buttons would act like everything works, I could hear the tccm click and a motor move but no four wheel drive. I replaced a vacuum hose to fix the problem. It is the one in this photo (
http://www.kylestubbins.com/files/im...zer_vacuum.jpg ) that leads down the center of the firewall. From there it does not go to a vacuum switch, but to a line near the transmission on the passenger side. Mine had come undone at an elbow near the right header/oxygen sensor. Let me know if that helps if not I will try to post a picture.