Likely culprit possibilities, in order of probability. This is a recent car that I am assuming has 60k or fewer miles, and that the description in the OP means that this only notably occurs between 70-80mph, and not above or below that range :
(1)- Tire balancing ('wheel out of round'). This is by far the most likely reason.
(2)- Warped rotor. This can be negligible at low speeds, but cause vibration at speed. This may be unlikely though, as you would almost certainly notice vibration during braking from those very speeds.
(3)- Engine/Tranny mount. This is easy enough to check, just take it to 75 and pop into neutral and see if the vibration subsides.
(4)- Suspension issues. Not very likely though, as this would probably have other presenting symptoms.
(5)- Hub bearings. This is not terribly likely either, as there is usually a 'graunching' kind of gravely noise when they start to go bad, and symptoms typically don't present themselves in such a narrow range of speed.
In short, it's probably #1 with a bullet. That's good, because it's the simplest thing to fix.
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