I'm bumping this back to the top because I have the same problem and am stumped.
One of my clients is 70 miles away (65 of it highway). I usually have the cruise set at 74. Some days are as smooth as can be, and some days the whole car just shakes.
My local mechanic has checked and rechecked everything he can think of and says everything is fine. He once thought it could be from the torque converter, but after I had the tranny rebuilt after it blew in June, it still shakes on occasion just as before. New tires, balance and alignment didn't help either.
