My vote is that you need a spark plug change. Because low RPMs at that speed is when mine started to bump around. Maybe you are mistaking what you think is vibration for a misfire in the engine.

 

Yes you can get a vibration at certain speeds if something is out of balance, you hit natural resonance at certain speeds or something, I dunno. I don't know how many times my wife cane home complaining her car shook, bot only at 30 mph...cleaned the snow out of the tires and it was completely fine. I had a jeep that would shake horribly but only between 50-60, turned out to be a bad rear u joint which surprised me since i felt it so strong through the wheel. My money is still on tires...hopefully something g cheap.and easy.