Another thought for this topic: condition of the crank pulley (harmonic balancer). Have you inspected the rubber on it for cracking and signs of fatigue/failure?
Following as I have a slight periodic something or another while sitting in the seat though it sounds perfect in the engine bay I can feel the vibrations.
Take a photo of the front face of it for us to see. Cracking on the rubber ring is an indication of age though it may be difficult to see. Heat and oil leaks don't help with rubber components.
You could also, carefully, pry with a lever arm between the pulley and engine. If the pulley moves easily then that's another indication of fatigue.
I couldn't get my phone in there with enough light by myself...and it is raining out. Maybe I can con, I mean, coax the wife into helping when she gets home.
Great photo and video. Nothing stands out visually. Engine looks to be very smooth and no visible vibration. The only thing I can say is that as durometer of the rubber changes from age, oil, and heat - the frequency at which it's supposed to function will change as well. This means it's no longer doing the job it's installed to do. Before you go replacing the pulley, try some of the other members suggestions.
Front O2 is OEM and 60K miles/10 years old. I think the rear is original...so 250K/24 yearsIf you haven’t replaced the O2 sensors I’d do that. I’ve been using the Denso parts for this for a while.
I’ve had them drift and on my first 80 I was getting unusually good fuel mileage for a while and they then gave a heater error and once replaced my mileage was back to normal. I change them out as part of my base lining. The EFI system relies solely on the O2s being accurate. If they aren’t the the ecu will just make adjustments to get the signals from the O2s it’s programmed to see. So it’s possible to run rich or lean and the system thinks everything is fine. That’s why it’s important to stick with factory parts here. Dropping in anther brand could introduce a problem that the ecu would be blind to. When I first used Denso brand I monitored the exhaust with a wide band to confirm proper operation.
I was focused more on you thinking you were running rich and the fact that after pushing the truck some it seemed to be running better. As far as the little stumble at idle, that could be a MAF sensor. I started including air straighteners with mine as there was a noticeable improvement on signal stability at idle afterwards.