You could always get a 10mm socket and pull the plate/mechanism fork off. There will be a little residue of gear oil, but nothing really to be concerned about making a mess. Just use some paper towels, newspaper underneath, and be careful of the paper gasket.
Pull it off, see how the fork slides the collar over, and minimum get a look at the axle splines to understand why it take some seconds to actually get some pull from the front axle. Then go ahead and check the vacuum.
You never know, you may have a situation like I had on my 87 XJ years ago. The fork broke and it broke so that I couldn't get 4wd. Luckily I had an ARB and at least 3wd.... :D LOL