Hi, adamantine,
Just a suggestion (from another guy trying to hunt down a sound/vibe). Have you sat the truck in park and revved it through the different rpms? I have a noise in my f150 that I can't find, I took it to ford and described it as a vibe happening at 30mph and 65mph... they went through all the drive line, tires, brakes... etc... found nothing. And just for kicks they revved it in place. It wasn't vehicle speed related, its was at about1000rpm and 2000rpm... and now I've found that if I run it to 3000 it does it also. Sorry I don't have any answers, but please check this out before you start digging into the diff.
I changed diff fluid, a wheel bearing, got tires rebalanced 3 times and alligned all for nothing... still haven't found my noise though. I'll post a link to my thread to see if you're experiencing the same thing.
https://www.ford-trucks.com/forums/9...vibration.html

 

I had this problem on an older blazer, either the rear yoke is spread (stretched wider than the ujoint) or less likely the pinion input bearing. Your ujoint will be fine but since the yoke it fits into is too wide it will move causing vibration. Unbolt ujoint and check to see how tightly it fits into yoke, if its more than 1/16 inch this is probably it. Bad part is you probably don't want to change it yourself as most require a puller and the nut has to be tourqed correctly. try it

Edit: it could also be the rear ujoint holder part of the driveline, in which case you would have to have it cut by a driveline shop and a new end put on, about 80$ good luck