Jeeps with the Select-Trac t-case (NP-242) do not have the vacuum disconnect front axle, only XJ's and YJ's with the Command-Trac (NP-231) have them. No ZJ ever came with the vacuum disconnect.

Try this next time it sticks in 4x4:
Shift to 2wd at a slow speed, forward gear. If the t-case does not shift to 2wd, then come to a stop, at the last couple MPH, put the auto trans in neutral (Stick, push in the clutch) and come to a stop in a straight line. Put the trans in reverse (Safe area, of course!) and back straight up a couple feet, then put the trans back into neutral and let it coast. 95% of the time, the 4X4 will disengage once the driveline unwinds without a load.

Causes:
The Select-Trac t-case uses a rather weak spring to disengage the 4x4. The t-case shift lever moves to the 2wd position, but it's actually the spring that moves the shift fork out of 4x4. Now, if your tires are SLIGHTLY off size from each other, either due to wear or mismatched sizes, or pressures, it gets harder for the spring to disengage the 4X4 from the driveline wrap due to the differing wheel speeds.

Cure:
What I would recommend is to actually inspect each tire, the pressures should be even, the sizes and brand must be the same, and the circumference must be equal to less than 1/2" difference between each tire, better under 3/8". Once they are, you will find it much easier to shift the Selec-Trac Transfer case.