If the tires have a lot of miles on them they could be out of balance. As tires wear their balance does change.
Bad tire, Check for bulges or bubbles on the sidewalls.
You could have lost a wheel weight or have a slightly bent wheel. (Did you hit any large pot holes lately?)
Ball joint possibly going bad or just very worn.
Since it is happening only at that high of speed I would look closely at the tires and wheels.
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[This message has been edited by Oreif (edited 02-01-2008).]