You guys are describing the exact same problem I have had for the past several weeks and it's driving me crazy! I do quite a bit of driving between Ottawa and Montreal, and two hours of that vibration is enough to put me over the deep end. Kind of ironic since I specialize in psychiatric research.
I too thought it was the spedo. However, this past weekend I was cleaning out the leaves from the air grill just a the base of the windsheild and noticed that the half grill on the driver's side is loose. I'm guessing that this could be the culprit.