4x4 not working
 

I have a 2000 f150 that the 4x4 is not engaging. I get the clicking as if it sounds like its engaging, I get the 4x4 indicator on, but it will not engage. I've replaced the following: both solenoids, actuator, vacuum harness. Check the fork, it's good . Vacuum reservoir holds pressure. Could this be related to my a/c system? I've noticed that my defrost blows when control knob is turned to Max ac as well as through the dash vents.

Not sure about the 4x4 problem, but the a/c issue could be vacuum related, panel/defrost door motor issue, or panel/defrost door malfunction.

my bet is vacuum for ac and 4x4

When I had a similar problem back when I bought my truck I recall the diagnostic process that called for checking the vacuum at the axle Vacuum there? your vacuum is good and you have a different issue. The lines are prone to melting if any battery acid gets on them. That was my problem. I just used a piece of vacuum hose to patch it and it still works great after 12 years. Check vacuum at the bottom first.