pull out your battery, pull out the 3 bolts holding your tray and your cable actuater is under there.all you have to do is get a set of vice grips on itso it stays pulled out put battery in and check if 4x4 works if it does then u got vacuum issues. if you look at ur t case it has 3 vacuum lines running off it, 1 is a vent, 1 runs to the actuater under ur batterie and the other runs to the drivers side firewall and connects to something over there, if either of the vacuum lines fall off or crack,or break ur 4x4 is gonna stop working.
This "vacuum thread" should be sticky http://www.s10forum.com/forum/f220/t...thread-327817/
"how to help narrow it down a bit. take a set of vice grips and manually pull the cable at the actuator and take the vacuum completely out of the loop. if the 4wd system does not work after this, then your problem is in the electronics, not the vacuum. it it works, then your problem is vacuum. to start diagnosing the vacuum system, make sure the lines going back to the t-case are intact and plugged in. if they are, then start the engine and make sure one of the lines will pull a vaccum. if it does, make sure its plugged into the valve properly. if that is correct then put it in 4wd (still with the motor running), and see if the line at the actuator is pulling a vacuum. this will narrow down which of the two lines may have a fault in it. the open air line does not matter, it could be un-plugged at the t-case for all it cares."