Well, it's no secret that YJ's have issues with their vacuum systems. I have had my share with both the Engine Vacuum and with the 4x4 vacuum. Below is a diagram I'd found of the Jeep 4x4 Vacuum system. My system didn't work at all, so what we'd ended up doing was to run a line from the carburator vacuum to the Yellow line to the front axle to lock it into 4x4 mode, and I would swap the hose to the Green line on the fron axle to pull it out of 4x4 mode. This bypassed the vaccuum switch on the transfer case. Initally this was a pain to figure out since when we found it first broke was when we were out in the snow on my ProjectJeep's first 4x4 trip. If you would like to talk about this, or the vacuum system, please write me here.

Since then I've changed from the old Vaccuum actuator system that changes the front axle from freewheel to locked with a solid "ring". I put in a Manual actuator that really worked a lot better than the old system. These Actuators have a cable that you put in through the firewall to a manual handle that moves the Ring lock into place, much like the original Vacuum Actuator shown below: