Personally I didn't want to lock mine in permanently. So I understand, that isn't for everyone.
You need to test the thing if you want to restore it to operation. You can test the vacuum motor diaphragm just by putting a hose on each nipple and pulling a vacuum. I just sucked on the hose, and if it had been leaking I would have been able to tell. To actually move the fork, you'll need some real vacuum. A jumper hose from the running engine will do that.
Also test the vacuum valve (they call it a switch, but it isn't electrical on an '89). Just pull it out and test its function. It's in the top of your transfer case. Not hard to get to.
Then pull out the entire harness and check it for leaks. I was positive mine looked good, but when I got it out of the cover loom, sure enough it had a hole burned in the green line.
There are lots of threads on this stuff, including diagrams. I'm new here, but I was able to find a lot of resources with a simple search. Your search term should include "CAD", which means central axle disconnect. That's what that thing is called.
At the end of the day, as long as my vacuum motor is good, I would fix it to original operation. That's the only expensive part at a little over $100. If/when my vacuum motor goes bad, I would spend that money on a Posi-lock system instead.