Nissan Quest - Vibration over 55 mph

I have a 1998 Nissan Quest. At 56 mph a vibration begins and shakes the entire vehicle body enough to make faster driving unpleasant. It gets progressively worse as speed increases. Between 65-70 mph, I back off and return to 55 mph where it returns to a normal smooth ride.

Have replaced motor mounts, all four wheel bearings, all four brakes, front rotors, rear drums, struts, rear shocks, tie rod ends, ball joints, steering rack, control arms, both CV assemblies and replaced all 4 tires, which have been balanced and checked several times.

The vehicle has 125K. The van drives beautifully at 55mph or lower. Any ideas?