[donut]

Can I throw something out? I mean, I'm far from an expert...

Forget the timing for now, 250rpm drop when you put it in gear. IMO, normal (ok, little high, but that can be adjusted. quit farting around adjusting until something works. Currently your going in circles.) My non lock-up trannys do the same, always have. Personally, I shoot for about 750-800 in drive. Forget what it idle's at in neutral for now. (unless it's like 1200 rpm, that would be another issue.)

Have you driven the truck without the OD? How does it act in just drive no OD?
Reading the symptoms you initially posted, you could be just lugging the engine down. With what your describing at cruise, if it was a standard, downshift and go.

The 700r4 may not be doing you any favors in this case. geezer#99 may be on to something when he asked for your rear gear ratio. 2.xx-3.xx with OD may NOT be the combo the engine wants.

What's your highway cruise rpm?

Back in the day my father had an old 6 cyl pickup. About once a month, we'd swap trucks. He was soo gentle driving it, it would load up and start fouling plugs. Let the teen drive it and "blow it out". he used to say it was night and day difference.

This may not be the issue, but currently it seems your chasing your tail, time to consider other possibilities.