I just went thru this with a new '84 GW. First, the on dash shifter is a vaccume switch with three vac. lines leading into it. If I remember right, two go to the transfer case "switch motor"and the third, red colored one goes to a vaccume "cluster" forward of the transfer case, it's hard to see/find. At the switch motor you have a vac line attaching at the back (from the dash) and another at the top of it which is a double line, One from the dash, the other from the hard to find vaccume cluster. you can follow this line to find the cluster. From here you go forward to the front axle where three vac. lines hook up, I can't remember where they all go to. Make sure all these lines are hooked up and not leaking. Start at the dash and test the red line, it's the vac in line which gets vac. from the "cluster" forward of the transfer case. This gets vac. via a red line from the vac. ball on the center of the firewall behind the engine. You can either pull the dash switch and check it now or go below and check the lines for vac. at the switch at the transfer case. If no vac. at transfer case check the dash switch and lines leading to vac. switch. Next check the vac. motor at the transfer case. I would pull the linkage off the vac. motor. It has an odd pin with a clip that holds it on to the motor push rod so fiddle around until you figure it out. The clip pushes up, rotating on the pin. Suposedly, this rod is moveable by hand but it's tough. Actuate the (dash)switch in both directions and you should see the rod move, if the vac. lines are all o.k.; including to front axle because any leak will keep it from switching, then the "switch motor" is probably bad. Try shifting the transfer case by hand using the linkage from the vac. motor you just removed, spinning the forward shaft to get it to engage and to shift. If it goes in to (4 wheel)gear remove the vac. motor., fiddle with it, bite the bullet and pay $125.00 to replace it through jeep, I couldn't find an aftermarket source personally, hook it all back up and test the vac. motor. if it doesn't move, you missed a vac. leak somewhere in the system. Next follow the linkage rod to its other end where you'll find a second linkage which is your high/low linkage. When in two wheel these should look parrallel to each other. Chances are, they are in adjustment unless someone has fiddled around with them. I sprayed these down with degreaser really well then sprayed PB blaster on them then, in four wheel, try to shift into low. You may need to roll the vehicle in gear a bit to get it to shift but do the actual shifting in neutral. If it doesn't shift into low still, go under again with a long screwdriver/breaker bar and try to move the hi/low linkage by hand. Your just trying to loosen it up so that it will shift. I did all this and got the four wheel system to work when none of it worked at first. The hi/low linkage was frozen from neglect and lack of use but I managed to unstick it and lub it up and it works fine now. shifts with ease. If you get this far and still have problems shifting (you should be able to manually shift transfer case from below), check the linkage adjustments. Next........ go to an expert!