As it is cold and snowing, i haven't had a chance to crawl under there and look.
The vibration starts at about 68 mph. If I let off the go pedal the vibration goes away. Does this sound like a rear drive shaft U joint?

 

It might be, you might want to buy a whole new driveshaft assembly any rebuild your existing one when it isn’t so cold and snowy out. (for next time).

 

i just went through that my vibration started at 60 mph or so and after a few weeks it got worse ended up being a rear u joint.

 

Check carrier bearing??

 

No carrier bearing, one piece rear drive shaft.

 

OIC so you have a short bed then

 

My truck did the EXACT same thing. I crawled under and everything felt fine. I took it to my buddies shop and he pulled the drive shaft and the rear U-Joint was nothing but red rust dust. There were no rollers in one of the bearing cups.
Replaced the U-Joint and its perfect again. Amazing that I couldn't feel any movement with the drive shaft on the truck.

 

my vibe started at 50, needed new ujoints here too.

 

Couldn't see signs of dust around the u joint caps. So it's been about 50-55K miles since the tires have been rotated and balanced. but they are worn even across, still look good. ( BF Goodrich original factory tires with 73000 miles on them.) Had them rotated and balance and the vibration went away.
Still have a drone sound at about the same speed that pulsates at cruise control speed.