I just resolved a similar issue with my '15 XLT/Sport Screw. My vibration happened, or at least I noticed it the most, when accelerating at 50 mph or more. I was spring-loaded to think I had a turbo or drive shaft problem. As part of their trouble shooting, the dealer road force balanced the tires. One tire wouldn't balance - they replaced it under warranty. The tech "eec tested for codes, condition normal", to rule out the engine or transmission. I even got a rental car at no cost. I have had other vehicles (other makes) in the past with a vibration or noise, and then had the dealer be dismissive about it, "couldn't hear it", etc. That wasn't the case at all this time, I was very well taken care of, thankfully.

 

Vince;
I know there are several model years between your '15 and my '07, but mine had a vibration that I could not find for a long time.
After trying many things, it turned out to be that my truck needed "camber kits" on the front. Mine had been lowered (it's a Roush stage 2) and there was enough adjustment in the camber bolts to make it go straight; but not enough to keep it from shaking.
It took a guy at a little "mom and pop" tire and wheel place about 10 whole seconds to figure it out. He looked at my truck from about 20 feet away, then turned and asked me "does that thing shake much?"
Ford dealer could not fix it, specialized truck shop could not fix it, old guy stands up to take a look at my truck from 20' away and nails it on the first try!!