You could try checking the black vacuum line against the firewall, towards the top of the engine bay. If you look at your distributor and look right there are two small plastic fittings for two vacuum lines. i think its the bottom one that controls whether the 4wd engages or not. I'm terrible at explaining things but i hope that helps, my 4wd stopped working after i took it in to get a tune up. I took a look under the hood and saw that had taking off the vacuum line and not put it back on.