I have a 4 inch rough country lift. It feels like its coming from right under the seat. I can look in my side mirrors and see the bed shaking at 70mph. The faster i go the worst the vibration.
I'm assuming that the truck is OK at slow speeds. Because that is way my truck is / was.
I forgot to mention one thing on my vibration issues. The part that extends the rear panhard track bar was put in the wrong position. The vibration was not caused by tires alone.
I would ask you to check all the items of the lift kit - anything that isn't stock and then go from there. Make sure that everything was installed the way Rough Country intended. It could be an item that wasn't installed correctly.
I'm sorry to say that I gave my mechanic a bit too much credit when I handed the task of installing the kit over to him. He IS a good experienced mechanic, but this was his first lift install. Sure he followed the videos and the manual, but there are always tiny details aren't mentioned or shown in videos and these things can get overlooked.