Wow. My last F250 bounced some at certain speeds, but the new to me truck bobbs up and down in the back from 65 to right about 76 MPH. Thankfully the speed limit is 75, so 80 is in the safe zone. Is the only fix some sand bags?
Both of my f150's did/do this. It didn't matter what tires or lift I had on them either. But mine does it around 40mph. But it doesn't do it all the time either, it's pretty random.
Im gonna guess rear tires are out of balance. my truck does it now any speed on the highway. my tires are crappy and hard to balance