ftyler,
in the family 1884 Olds 98 Oldsmobile - rwd , we had a similiar problem... it would vibrate between 50-60 or so mph... it turned out the rear(axle end) 'u' joints and the metal clamps were slap worn out... and it was jsut a vibratin' around back there and was close to coming loose
Pop replaced them with new joints and life was good.
Anyway, it is something to check...
-W
