Okay. Pulled the plug. Huge pita, glad y'all didn't tell me that in advance.
The plug itself looks fine, wires aren't cracked at the base like some photos I've seen.
Bad news. It looks like there's ATF in the plug. I've seen this is bad, but what's the fix?
Pardon my ugly mug in the photos, it was hard to get a camera up in there.

 

Crap! that is bad, not talking about your mug...

 

Sure looks like the seal in the connector has failed. I think the solenoid will need replacing but I don't want to make that call. We need a transmission expert for advice.

 

Where is a good place to source a solenoid pack, and does anyone have an OEM part number for it?

 

Another crappy update.

After cleaning the plug, the truck drove well for about 300 miles.
Today, the truck slipped out of OD at about 70, I was able to regain it by dropping to ~55 and turning the OD off. Truck popped into 3rd, I got up to 65 and turned the OD back on and had no issues after that.
Later, after the truck was parked, when in D the truck was stuck in first. I had to restart the truck several times for it to drive like it should. The first time it started, my odometer was blank, and I had to knock on the speedometer housing to get it to come on.

My codes are 622 O, 452 C, 542 C, 624 C, and 637 C. All of the C codes repeated themselves once.
622 O, 624 C and 637 C all sound like a continuing issue with the solenoid body plug. 622 is shift solenoid #2 circuit fault, 624 is electronic pressure control solenoid circuit fault, and 637 is trans oil temp circuit over voltage. I can confirm from a secondary gauge, my trans oil never exceeded 150*f.
Pretty sure 542 C is because my rear tank doesn't work. I flipped it on recently to depressurize my fuel lines to change the fuel filter, so I'm 90% confident that's why that showed up.
452 C is a VSS code. I haven't checked my rear diff yet, but after the issue with the odo tonight, I'm thinking my PSOM is failing.

Thoughts and opinions on how to move forward with this appreciated!