I believe that is the vacuum to engage the front axle when in 4 wheel drive.
This sounds like a YJ jeep issue, I eliminated the vacuum actuator by installing a heavy duty choke style manual cable attached to the vacuum actuator arm, that way I can lock the front diff when I wanted. Make sure you cap off the vacuum line, otherwise your engine will run poorly.
On the passenger side firewall, above the blower motor. There is a vacuum line attached to it that connects it to the top of the engine and an wires attached to it on the front.
I believe that it's just above the Front Right spark plug coil. It has a vacuum hose attached to it.
One of the vacuum hoses is probably broken. I had the same problem on an 89 vandura. Turned-out to be that the hose was gummed and couldn't hold the vacuum. It was a hose connected to the vacuum reservoir (ball-like object attached to the hood). Good luck.
diagram from front vacuum motor to transfer case of 1993 Jeep Wrangler
M. Night Shamalyan plays in the village (Park ranger) and signs(The fathers friend) Zack Snyder appears in dawn of the dead as one of the machinegunners in front of the capital that open fire during a news report(this scene is in the introductional credits)
It is under the drivers side front corner. It is rectagle rather than round like most. You can see it by looking up under the left front corner. It is held in place with one bolt and has one vacuum line attached.
Front of the frogs mouth.
THE VALVE IS UNDER THE HOOD, ON TOP OF THE ENGINE, IN FRONT OF THE PASSENGER WITH 4 5/8 HOSES AND ONE SMALL VACUUM LINE ATTACHED.
both, front engages by vacuum, transfer case is electronicaly shifted
there is an electric vacuum pump on the front pass wheel well
On the underside of the hood near the front there is a diagram of your vacuum hoses and where they go.