2002 XLE w/100K miles. Intermittently, at highway speeds, the front end vibrates significantly. There is no grinding or other noise accompanying the event, which eventually stops and may not recur for an hour or more. The driver is not stepping on the brake at the time of the event, and slowing down does not stop the shaking. The van has been in the shop (dealer service) including yesterday. They advise the front end is fine, as are the brakes. They drove it an extended period, but no shaking occurred while they were testing it. Another shaking event occurred this morning. Any suggestions?