proFISHent - Should you wish to repair your 4x4 vacuum system please take a look at this link. http://oljeep.com/87_yj_4wddiagram.jpg
I found this a while ago and it helped me troubleshoot the 4X4 system on my 87 YJ and I found the system is simple as dirt. No mysteries.
My problem was a broken vacuum line which I spliced using a short piece of windshield wiper hose. Cost? -0-.
Since then, my 4x4 system has operated every time since and I have found it to be 100% reliable in sand, mud, and snow for the last 2 years.
I don't have anything against a posi-lock other than the price and the need to do two things to do when you want 4x4 operation rather than one thing to do with the vacuum system.
A few troubleshooting hints if you decide to go that way;
The vacuum switch is located on the top of the transfer case and has 4 hoses, which are;
1. Blue - goes only to the Vent/Check valve. (Mine sort of hangs loose but I tie-wrapped it one of the other hoses)
2. Green- goes to one side of the CAD on the front axle.
3. Yellow-goes to the other side of the CAD on the front axle.
4. Red-this is your vacuum pickup and goes to the manifold . (This was what I repaired with the windshield wiper hose)
Black hose- goes from the CAD to the 4x4 switch on the firewall. (I think is a dual line with the yellow line in sort of a dual connector - its been a while)
The yellow line is tough to see on the link diagram but it goes from the switch on top of the transfer case to the CAD. ( see #3 above.)
I used a brake bleeding hand vacuum pump to test the CAD (vacuum actuator) on the front axle. Hook the pump to one side of the CAD and then the other side to see if the CAD moves in and out or leaks.
See - the system is simple as dirt with no mysteries if you have a diagram, theory of operation, and where the parts are located.
Hope this helps - yours could be a zero fix also.