Does it go away @ 75..80+? If yes, it's a balancing issue. I'd try to determine the defective tire (or wheel?) by first swaping left side front/rear - if it goes away or at least you feel shaking but the steering wheel doesn't twitch - it's the (now) left rear tire. No changes - swap on the right side.
I know it sounds labor intensive, but may save your from replacing all 4 tires (or wheels). On my other car I had a tire that produces horrible road noise but visually was ok, so it took me two iterations before I was able to pinpoint the guilty tire and swap just it - well worth the effort given it was a ridiculously expensive - $350 for 17" !! - run-flat dunlop.