my 99 se duratec mtx has started to do the same thing, but it began only after i had my tires rotated...im thinking they're not balanced, which is probly the same problem you're having. i just turn up the radio so i forget about the wheel shake (haha). but i do believe for me it's either uneven tire wear, under/over inflation, or like it may be in your case...they're off balance.
