Ok, so here's the remainder of the story. After a number of hiccups, some leading to visits with Eides's diesel shop, the guilty party ended up being an Exhaust Gas Back-Pressure Sensor. Kind of an odd thing overall, but I did a bunch of internet sleuthing to arrive at the most likely problem and kind of pushed them in that direction. Note...the guys down at Eides are great to work with, but I don't think this is something they have seen very often.
Anyway, got a new sensor in and some XX months and 8k miles later and she's running fine.
Still thinking of swapping it out for a newer ride this spring. It's been a great ride for some 10 years and 130k miles, but I'm ready to upgrade. If you're in the mood to donate, just send me a pm!
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Note, the guys at Eide stuck a scope up that F250's derriere and turbo enough to not see anything that causes concern with the exhaust itself. She's got a complete clean bill of health.