I had that problem in a chevy once and it was custom made 3500 stall converter. I would get my bad vibration around 50mph. Sometimes I had it and sometimes I didn't. I could fix it, at speed, by slipping the tranny into neutral, rev up the engine a couple of times, then slip it back into drive and usually it would clear up. I even took it back to the tranny shop to have it rebuilt. They cut it open and couldn't find anything wrong with it. I put it back in and the vibration was less but still there. Fixed the problem permantly by replacing converter.
Not sure if this is your problem though but wanted to let you know of my experence.