the transfercase is manually actuated by the big lever in the cab. those vacuum lines come from a little switch on the t-case and go to the CAD (central axle disconnect) on the front axle. The CAD is vacuum actuated.
I suggest ripping out all those problematic vacuum lines, plugging the source, and shifting the CAD so that it is engaged all the time. This will not affect any of the 4wd functionality of your truck (Jeep did away with that stupid CAD in the early/mid 90s), it'll only make it infinitely more reliable. :thumbsup:
See here for steps to defeat the CAD once and for all:
viewtopic.php?f=9&t=18415