I have an 87 I'm having trouble with 4wd engaging it's not in linkage I've checked vaccum lines they look fine up to the front axle there is one line under the truck that has a plug on it that moves in and out. And one is open ended I put a bolt in it thinking the plug had come out but still nothing. I need help I can figure it out and I can't find a vacuum switch on a transfer case just where vaccum lines plug in please help