Why all the 4wd vacuum tubes

My son has acquired an old 1992 4cyl Jeep wrangler YJ that had many disconnected lines/wires/tubes that we got mostly figured out and basically connected to the point where it's running fine. One area that seems to be functioning OK is the front 4 wheel drive system that when the manual lever is placed in gear (up on blocks), even though all the vacuum tubing is messed up and not totally connected, looks to be engaged because the front wheels are turning. This is fine with him because he only wants to use it in winter on snowy roads. My question is.. is it really in 4WD or just playing possum ? That vacuum module on front axle.. does it possibly engage something when working correctly that may be stuck in the engaged position ? What's going on here