Have you tried sitting the rear axle on jack stands and running the truck up to speed and seeing if the vibration is in there? If doing this, block the front wheels, and make sure the rear wheels are not turning when shifting gears! Use the brakes to bring things to a stop. Oh, and make sure you're in 2wd mode!

 

Nope, I for sure haven't done that. I never tried doing anything like that. Seems a bit hairy. I will look into it. This would help narrow things down. Which is what I'm trying to do. Hoping to start with the small things first, then go from there.

 

I've done it a few times. And, yes, it can be kinda hairy. Best done with a 2nd person around to watch, but don't get close to the wheels when you run them up to speed. And, DO NOT GET UNDER THE VEHICLE WITH THE WHEELS TURNING! If the driveshaft is a concern that it might be bent, you should be able to see that without getting under the truck. Just let the wheels roll at a slow speed, and focus on the driveshaft and a part of the truck and you should see a wobble, if it is bent.