Welcome to the board, Matt! /ubbthreads/images/graemlins/smile.gif
I've ridden in this car, and this vibration is kind of funky. I didn't feel well enough to drive it myself, but the vibration threshold does not appear to be directly tied to engine speed or road speed, it seems to be more about load.
[edit]To clarify, you can run at any rpm/speed, and there won't be a problem until a heavy load is applied.
Once occuring, the vibration appears to be tied more to road speed than anything else, suggesting a driveline problem. A bad/binding/dry u-joint or c.v., or a problem with the carrier bearings were my initial feelings, but I also can't rule out a front/center/rear diff or internal transmission issue. I haven't been underneath this car yet, but Andrew and Gary are working on the problem, and I'm sure they'll find it.
In the meantime, I'm sure your suggestions are welcome, and appreciated.