2015 sr5 weird squeak/vibration around 50mph

so, I lifted my truck almost two years ago and it has always had some vibration around 50mph. Recently it has gotten worse.. the bed is shaking pretty bad and can kind of feel it in the floor. Today when I let off the gas getting off the high way it started squeakin kind of like a fluttering squeak.. cant seem to get the truck to do it again currently. In the past garages have greased my u joints/drive shaft now that I remember.. I've gotten under the truck and everythinf seems pretty solid there, no movement. Anyone else think if could be the u joints?

 

Does it go away if you switch into 4WD?

 

No. Same as in 4WD.

 

It’s possible that the U-joints are causing it.
Have you checked the center carrier bearing?
The one that is bolted to the center of the truck.