Is there a switch or relay that could be keeping the 4 wheel hi from working on a 2000 Chevy S10?

There is a vacuum actuator under your battery. When you push your button it activates your transfer case motor. Then the vacuum actuator is attached to a cable that goes to your front differential. These vacuum actuators have been known to go. You can either replace this unit or look for something called "4x4 posi lock." It replaces the vacuum actuator completely, but it is a modification and a handle will go under your dash. You would push the button as normal then pull the handle. The benefit of this would be that it is more reliable and you can use your low gear in two wheel drive if you need to by not pulling the cable...hope this helps....goodluck