UPDATE! And, as usual, a mixed bag...
I brought it back to the transmission shop for it's break-in/warranty check up (it did have a leak...) and asked if they would cut me some new transmission cooler lines (where they go into the driver's side of the radiator) since they weren't happy with the new e-fan shroud/assembly I had wedge between them and the radiator. I also asked them to check the exhaust/catalytic converter since they had to remove it to service the transmission and it was now rattling...
When they called and told me the exhaust noise was coming from INSIDE the catalytic converter, I had 3 options. Remove it and send it back to Rock Auto for a warranty replacement (request, no guarantee), gut it or replace it with a test pipe. Since I wasn't sure what the issue was and have had issues with Rock Auto, never mind the shipping cost to send it back, I told them to either gut the converter or replace it with a test pipe.
The leak was apparently coming from the transfer case and they had to wait for a seal which was backordered so my truck sat for a while. They called me the other day and said it was ready so I picked it up yesterday.
The gentleman that did the exhaust said the converter was wrecked (internally). Apparently the substrate on the rearmost assembly came loose and had twisted/shifted sideways so it was effectively blocking the exhaust. The substrate in the forwardmost assembly had broken loose from it's mount/welds and shifted back several inches, which he possibly attributed to the massive increase in back pressure from it being nearly completely blocked. So, effectively it was a mechanical defect with the converter itself that was causing me performance issues, NOT the running condition of the motor itself and frying/clogging it. I asked, rhetorically, if that would cause performance issues and got the response I thought; of course it would, the motor was being choked by a significantly restricted exhaust.
That was "good" news...I guess? I was looking forward to taking it through the pass as a test to see what kind of difference it would make, if any, and jumped on the interstate. After going through it eastbound, which has a much more mild grade, I took the off-ramp and flipped a u-turn to go east bound and started on a significant grade at approximately 15 MPH. Slowly but surely it pulled to just over 65 MPH and at the steepest part of the grade, where it completely fell on its face and could barely hold 50 MPH it dropped to JUST under 65 and was hardly struggling.
So I went through the same elevation/pass at almost 15 MPH faster than before. I'd call that a win and say the converter was absolutely the cause of the issue. I'm likewise positive that its effect on my gas mileage was pretty significant too, time will tell on that.
I took my usual desert shortcut and it ripped through it like it's never done before, which made me wonder how long the converter has been defective/acting up. When I went to exit the desert and get back on surface streets the 4WD button on the overhead console, which has ALWAYS been janky, got stuck inside, and even after prying it out with my knife would not reset in its proper position...UGH!
So when I got home I started futzing with that and it did NOT want to come out. I don't know if one of the springs came out of position or what, but it's never been particularly solid. I MIGHT convert to manual axles and just be done with messing with that. There's no issue with the Low Range button...at least for now!
I then went into my glovebox to put my receipt in it and an aquarium spilled out onto my floormats. We had several days of torrential, near Biblical rain here and there's obviously a leak somewhere that I need to chase, so that's on the list now even though we rarely experience rain here, I still need to see if I can figure it out...
I didn't further inspect it until later when I wanted to see the handywork on the new transmission cooler and lines. The new cooler is a very nice and rugged looking B&M unit I believe, and as soon as I popped the hood I realized they hadn't touched the cooler lines. WTF...
I called the shop and spoke to the manager who originally did my drop off paperwork. The techs/mechanics had gone home for the day, now I'm just waiting for a call back. So now I'm gonna have to drop it back off for work that should have been done the first time, and wait...
I stopped by my other local independent mechanic who I asked for an estimate to R&R my heads for a new set of sexy ones like
@gaz and I were discussing. He hadn't texted me back because he's been busy AF and I'm waiting to hear back on that. Any delusions I had of doing that myself have recently gone completely out the window as I (re)injured my severely gnarly back (my lumbar vertebrae, L1-L5 are completely fused with 4 rods and 10 screws in addition to my left SI joint, NOT to mention several other significant orthopedic trauma surgeries...) and even this odds and ends stuff is gonna be a challenge...
Other than that everything is swell!