Vibrations at speed are often caused by rotating parts. In your case, I might be looking at the torque converter. The next time it vibrates, try taking it out of overdrive to see if that causes the vibration to go away. If it does, then it may be a torque converter or transmission problem.
More info on solving vibration problems here: http://www.ericthecarguy.com/faq/determining-the-causes-of-vehicle-vibrations
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