Odd HVAC problem

99, 4.0, aw4, 4x4, 153xxx miles.

I'm having issues with my HVAC vents. When turned to face or defrost, a door seems to be sticking shut and I only get loud whistling. When turned to foot, it works great every time, and when turned to blended foot/defrost it seems to help the defrost work occasionally. Sometimes I can drive for hours without issues, other times the doors are constantly slamming shut in the vents.

-Today, I replaced the vacuum lines running from the vacuum canister behind the bumper all the way to the intake manifold, which did not help.

-I checked all of the arms on the actuators, and all are intact and appear to be functional when switching between face/foot/defrost.

At this point, I'm thinking I have a weak/failing actuator, or I am losing vacuum somewhere between the line in the engine bay and the actuators, allowing the door to slam shut. If this was the case, why would the foot door function flawlessly? This leads me back to faulty/weak actuators.

Do the face and defrost doors share the same actuator?

Where is the vacuum source for these actuators? It seems it has to be the tiny vacuum line coming off the main line between the cruise and the intake manifold.

My main concern is safety, monsoon season is here in AZ and it gets a little sketchy on the freeway with a foggy windshield.

It sounds more like a control issue, but you can check the vacuum actuators by unplugging the and then applying vacuum with a hand pump and then seeing if they hold or if they leak down. They should hold 10-12 inches of vacuum nearly indefinitely. Not positive, but I do believe the cherokees use a mechanical vacuum switch, if the actuators test good, I would say it most likely the switch.