Those aren't vac lines, they're the vents for the axles. As you drive the axles heat up, without the vents they would build pressure and pop seals.
The Central Axle Disconect(CAD) system in the earlier XJ's and MJs was a vacuum motor that actuated a fork in the passener side of the axle. The fork would push a collar, locking the inner shaft and outer shaft together, engaging the passenger side hub to the differential.
Jeeps with the Full-Time 242 T-case never had it, and it was only on the early 231 models. Newer models, I think 95+, have a solid axle shaft and the d-shaft is engaed all the time, it spins even when you're not in 4WD.
The easiest way to determine if you've got a CAD axle is to look. You'll see a big black thing on the passenger side of the axle, it will have vacuum lines going to it that are controlled from the t-case. usually when they fail they just ned cleaned, and you can "trail fix" them by taking off the vac motor and flipping the fork around to engage the collar, then putting the motor back on.