while we're playing "what is it", right below the part you highlighted, in the lower center of the picture is what appears to be a small rubber hose sticking straight up with what looks like a wooden dowel stuck in the end of it. Does anyone know what that is and what function it serves. I've wondered about that for a long time..
It's probably a vac line from the Tcase then, if it has another line coming off of it near the intake going into the firewall, then that's what it is (that part that comes off would be going to the indicator light).
For the record, one of the tubes from this vacume thing runs to the fuel area, like on the intake manifold.
Ok, if you haven't figured it out yet, here's how it works.
In your picture you have circled the main vacuum check valve on the tube harness for the front axle disconnect. It serves the purpose of keeping the vacum in the engine when you don't have the TC engaged. The hose that runs back over to the intake manifold gives it the needed vacuum to operate the disconnect as the manifold is always in vacuum when the engine is running.
Handicapped license plates are not for mentally handicapped people -
"Man has not yet evolved an inch from the primordial slime that spawned him." Mark Blankfield, Jekyll & Hyde... Together again