94 3500 4x4 gmc. Transfer case wont work?

[INSIDIOUS '86]

So my old mans 94 gmc dually extended cab 3500 runs fine and is great all around. It usaully sits for months at time before being driven. No matter what we do it wont go into low or high gear for 4wd. Basicly its not doung anything. Any of you guys have this issue? Not sure if its vacuum controled or electric sylinoid or mechanical? Thanks for any help guys!
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[64swb]

Does the transfer case actually shift into 4hi/lo? I'm assuming the tcase actually shifts into either one of these ranges but the front doesn't engage. If this is the case its most likely the front differential actuator. It is electronic and they are notorious for failing. You can get a replacement stock type one, or like I did there is and aftermarket cable setup you can convert to so you have a pull cable under the dash to lock the front end in. I believe it is made by posi lock? Can't remember, I got mine through summit though.