The vehicle had low airflow out of the dash vents. When I replaced the mode actuator, I heard it click like it was skipping gear teeth... meaning there was a mechanical problem in the linkage someplace.
I started by exploratorily taking the center stack and bits and bobs of the dash apart.
I took lots of photos. Here's the album:
https://imgur.com/a/60GRIpp
I found that I could see one part move with the mode actuator. It would wind up, then kick back when the mode actuator skipped teeth... Ok, so that means there's a mechanical problem in the airbox that has the mechanism jammed.
That album also shows alternator drip shield installation and pulling junk out from under the passenger seat.
//
Knowing this, I then removed the upper dash. The upper and lower dash, the steel structure underneath and the dash wiring harness were all built up on a bench and then installed into the vehicle as an assembly. Removing the upper dash isn't awful... it's just not super fun since I was doing it out of order.
Album of that:
https://imgur.com/a/ztwnsf8
(Yes, I also received Steel Armadillo super sliders that I ordered... more on those later)
The highlights:
Upper dash removed:
As long as I plugged the instrument panel back in, the vehicle would run and drive like this
The culprit found:
Of course it's down INSIDE the airbox:

//
The dealership got me the part number for the air handler assembly. Of course it's discontinued and neither Mopar nor any dealership in the network still has one. Summit and MoparPartsGiant still list it, but both canceled my orders due to inability to get one. I found one at a junkyard about 45 minutes away, so I snagged it.
FYI:
Car-Part.com--Used Auto Parts Market saves the day on this one. I found the unit under "Heater Assembly"
Here's the album of that adventure:
https://imgur.com/a/pOCJsQH
Highlights:
The HVAC assembly is surprisingly large:
I could not just install the junkyard one because it has a big crack in the duct from the blower housing to the AC Evaporator
The intake air comes from either the cowl intake or the interior of the vehicle. The air goes through the evaporator first. The evaporator is HUGE. Then the air goes through or bypasses the heater core, depending on what temperature the control is commanding. Then the air goes into the air handler, which has all the flappers and valves that control how much air bypasses the heater core and what vents the air comes out of.
There are three main housings: The intake housing, the blower & evaporator housing and the air handler & heater core housing. I found out later that another reason I would not be able to use the junkyard air handler is that my vehicle has an electric auxiliary heater, and thus the shell halves and center divider of the air handler are DIFFERENT from the unit without the auxiliary heater. I'm not sure if the aux heater is a default with the diesel or if it's part of a cold weather package. My vehicle has a block heater as well.
There are 7 screws that hold the air handler to the evap housing. TWO are really difficult to access while the unit is in the vehicle. However, removing the unit from the vehicle requires blowing down the AC... which I did not want to do.
Video of the mode actuation mechanism operating:
https://imgur.com/GRWBx7G
A couple of the air handler by itself:
Also, as I was taking the dash apart, this fell out:
That got me to reading about the recirc door gear failure... my vehicle, being an '07, had the first design recirc door gear. The junkyard assembly had the second design recirc door gear. Score! Two problems knocked out with one operation.
At some point, one of the vehicle's windows was broken. I found shards of safety glass under EVERYTHING I took apart in the interior, including down the bottom of the air handler. Broken glass down the windshield vent could have FODed the flapper valve to the point that it broke and jammed. The driver's window has a different manufacturer's mark than the other three. I also found a
toggle bolt (type of drywall anchor) in the bottom of the air handler, so that could also have FODed the flapper.
Here's it is with the dash out:
Here's the air handler split in half and upside down; the rectangular opening is where the heater core goes, but it at the bottom of the unit
Here's what I had to do to get to the back two screws on the air handler... not so much fun.
Broken flapper
How the flapper and drive gear should look:
Vehicle with the air handler removed:
Bad lighting for these photos, but here's the lower dash assembly out of the vehicle:
"Installation is the reverse of removal"
ALLLLLLLL of that for a $3 plastic part... but it works now.
Fixes include:
-Repaired mode actuation linkage
-Replace recirc door gear
-Almost to the point of replacing the damaged terminal in the production break in the vehicle harness
-Rekeyed ignition lock to match door lock. (*)
-Broken hazard light switch and broken steering column shroud replaced (**)
-Vacuumed and cleaned under all the interior components
Since I started taking the rocker covers and door sill trim apart, my next fix will be the airbag impact sensor code... but it sure would be nice to verify which side impact sensor is #2 before I tear into the wrong part of the interior.
(*)The ignition lock recall has been done on this vehicle, but it had a different door key than ignition key. I'm pretty sure the door key is still the one in the Mopar database, so since I had the ignition lock out, I had a local locksmith rekey it to match the door key and give me 3 more keys so I have some spares. The keys also have RFID chips in them, for which the vehicle has to be programmed. The remote that came with the vehicle now no longer starts the ignition, so I'll have to go to the dealership to get that done

. Local stealership wants $270/key and another $90 to program the vehicle, resulting in me not needing that for a while.
(**) The hazard light switch has never worked since I've owned the car. Since I had everything apart, I removed it. The switch was mechanically broken. It's been discontinued by Mopar, no supplier has the factory unit in stock. There was an aftermarket unit made for a bit, but it's NLA as well. I found one in the junk yard in the tow where my dad lives. My steering column shroud also had a broken screw boss, so I replaced it with the shroud from the junkyard vehicle the switch came from.