As if a constant vibration isn't hard enough to diagnose, mine comes and goes. It will start around 70km/h and get consistantly more violent as the vehicle speed increases. It will be there for a few days, and then bam. Nothing. It was about a month ago the last time it happened, and it lasted for one drive. This time it stuck around for a few days and was gone again...

Sounds like an ignition miss to me.

Or some snow on one of the wheels that melts when it'* ready. Do you have snow currently?

Rediculous amounts. I never even thought about that.

As for an ignition missfire, I could go 0-60 WOT and it will be perfectly smooth. Just up and after 70 it starts vibrating.