4WD not working or working intermittently

What is your question?

Dealeer tech says cannot fully diagnose problem because of front mounted hitch receiver but is most likely leaking vacuum line or pump not working. Local mechanic was able to determine that there was no vacuum to the front diff. but 12-13 lbs from pump. Bypassed vacuum lie to diff and now has 12-13lbs at front diff. Dash 3WD light not coming on, don't know otherwise if 4WD is engaging, but doubt it. What to do next? With vacuum to front diff.which was not there before, what is most likely problem now?