The only thing the vacuum modulator does is lock the axle on that side to itself so it will turn in the differential. without it locked in, the wheel end of the axle will freewheel. With both front wheels off the ground, I'm not sure you will be able to tell if the gear is locked in or not.

The vacuum only pulls the sliding gear in or out, depending on which vacuum port has vacuum. Once it is slid in the direction the port dictates, there is no need for the vacuum to hold it in place, but I believe the vacuum remains.

One of the biggest issues with this setup in the vacuum lines. They are thin steel lines with rubber hose ends, and the hoses go bad and the steel lines rot out.

You can manually lock the gear in by removing the vacuum unit, sliding the mechanism into the lock position, and shifting the vacumme lever to the side making it hold the slider in gear, then reinstalling the vacuum mechanism. To disengage the 4x4, you have to remove the vacuum mechanism disengage the slider, center that vacuum mechanism and reinstall it.

Pulling the cover to inspect a differential is never a bad idea, but unless its really broke, your probably not going to gain anything concerning the 4x4 function. Gene