I have an '03 Expedition with a Ford Accessory bug deflector and I really haven't noticed the "raining cats and dogs" condition. Did you purchase yours from Ford?

 

I have the Ford bug deflector installed. No hood vibration at all.

 

To all those with hood vibrations or shakes, have you tried to adjust the rubber stops? I'd have the dealer body shop adjust the fit of the hood. I have a hood defector from Ford and at what ever speed the hood does not move. If one hood doesn't they all shouldn't... it's got to be the fit of the hood.
JE