Lol I missed the 80 mph part okay I'll get it up to 80 here in Sf I have gone 80 uphills before for long periods of time on a trip to Socal didn't notice any vibration if that helps. Will make note of it next time I'm on a hill that a can do 80 mph.
This may or may not be helpful, but I felt a vibration under my feet as I drove at higher speeds on the highway. Could not figure out what it was, thought maybe something was loose in the drivetrain...Then I stumbled upon an answer by accident...mine ended up being the fact that the carpet was not flush with the floorboard-it sat about a mm or 2 above and when there was engine power it would vibrate the two together. I had to put a little velcro (just the non-fuzzy side) under there and it stopped.