I had assumed the vibration was constant...
Have you jacked the car up, placed it on jackstands, and tried jiggling the wheels (all 4) to see if anything is loose?
Once I had a CV joint start to go out on me and it would make a weird kind of vibration at about 70mph. It would get worse as I got into the gas and better when I let off a little. It never made the clicking when you turn (as is the normal telltale sign).