i have a 98 blazer, 4 wd drive stopped working, i checked the vaccume and there was none to the accuator but pulled the vaccume off the switch by the transfer case and the switch had vaccume, also bought a vaccume line and used the vaccume from the brake booster and the accuator pulled in just fine, but it still wouldn't engage into 4wd, it should of engaged into 4wd drive with vaccume from the brake booster pulling the accuator in correct? or does the switch need replaced?

 

Sounds like the switch needs to be replaced, sure, but the fact that it did not engage even with full manifold vacuum from the booster source means that there is another problem. It likely is a cable problem. Try working the cable back and forth while spraying some WD40 or equivalent light penetrating oil into the end of the cable to see if you can get any more movement out of it.