Are you sure that it is the engine vibrating? I would think that an engine vibration would occur in every gear once it reaches the same load and rpm as what occurs when you hit "45".
Vibration due to tires/wheels usually does occur at a specific road speed in my experience.
Carl

1952 5-window - return to "as built" condition | 1950 3100 with a 235 and a T-5 transmission