RE: Heater/AC will not blow correct
I would say it is a linkage problem if you can hear the vacuum and motorized movement when changing selection.
This happened to my 93 camry. Everything works except for the air flow direction. Sad thing was, we had to tear down the dash to get to the A/C assembly and voila, saw the linkage arm came off. This repair costed me a couple hundred bucks mainly in labor because of the entire dash removal. Otherwise, it would have been a quick 2 second fix.
i am almost sure that for you to repair this possibly similar issue on the Durango, the dash will have to come out to get to the A/C core assembly unit. My buddy removed my dash on the Durango to replace my heatercore and it was a 4-6 hour tear down job.
If you have to go this route to address your issue, this is a good time as any to inspect and possibly change out your heatercore, otherwise, you may end up tearing down the dash again to replace the heatercore in the future.