Follow up.
The leak was at the electrical connector. It is called a "pass through" seal.
There are two ways to approach this. 1. JUST replace the outer seal. or 2. Remove the pan and the internal wire harness and replace the entire pass through seal assembly, which is two pieces plus an "O" ring. There is actually a third way, which is buy a new internal harness, which will include the pass through seal.
The first procedure is pretty simple if you opt for that. Problem is, you won't know if that fixed it until you are done.
Here is a short clip of the removal. New piece just pops in its place.
https://www.youtube.com/watch?v=ykYvOWujTyk
After a bit of research, it appears that the success rate is about 50%. I had already planned on a second drain and fill, because I don't have the means to do a complete pressurized flush. Figured I would just pull the pan and do the entire thing.
This company offers a kit with all three pieces.
https://fleeceperformance.com/media/...M-LCT-SEAL.pdf
They also provide pretty good instructions.
I ordered one. It was "delivered" to my office on December 23. Problem is, the substitute postal carrier just dropped it on the porch (tracking info said "delivered at or neat mailbox") and someone walked off with it.
Got a second one. Had them ship this one UPS as our UPS driver ALWAYS comes inside the building and drops packages off at the front desk (as does our regular postal carrier.... unlike the moron sub).
Here is a video of a guy replacing the internal wire harness.
https://www.youtube.com/watch?v=ZYnQgRvSXqk&t=1463s
Super easy job if the trans is upside down on the bench. Not so easy in the vehicle. As you can see, the guy struggled a bit with it.
I could NOT get the exhaust hanger loose to get it out of the way without a torch. So, just worked around it.
If I were doing this again (won't likely need to) I would just buy a replacement harness. They are available on line at anywhere from $120 to $700 depending on where you source it, and if you are buying OEM. Why would I buy the whole harness? Using the kit that I used, I had to remove each wire and pin out of the plug assembly and then install into the new plug assembly, one at a time; all 24 of them. The retainers are really tiny. I have a dental pick I use for stuff like that, but couldn't locate it. Ended up using an ice pic, which was less than ideal. Was really afraid bending or otherwise damaging one of the pins. Also was afraid I would get a couple mixed up. No room for error.
Bottom line is that I got it done, and apparently got everything where it belongs, as the truck drives and shifts as it should. Just increased the pucker factor.
The fluid (Castrol TransSynd) is expensive. Glad I bought a 5 gallon bucket. Having pulled the pan twice, I ended up using 18.5 of the 20 quarts!!!!!
While I was doing this messy job, I decided to replace the oil cooler lines. What's a little more oil dry? One of the cooler lines had just started to drip, so I knew the other wasn't far behind. There is simply no way to pull those without making a mess. Pretty straight forward, just messy.
Best news: no more leaks!!!!
All new belts, idler pulleys and tensioners, as well as an oil and filter change and full chassis lube. Believe it or not, this thing still has grease zerks!!!
Bad news: the battery is definitely getting weak. Starting to crank slowly. This will be my fourth new battery in two months.