Vibration over 58mph

I have a 2000 Nissan Quest. At 58MPH a vibration begins and shakes enough to make faster driving unpleasant. 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 and all 4 tires which have been balanced and checked several times. All of these parts had been worn and in need of replacement, the vehicle has 170K. The only thing I have not repaired is the rear stabilizer bar link bushings. The van drives beautifully at 55mph or lower. Has been a real puzzle and continues to be…
Have noticed that when there is weight in the rear, or going uphill the vibration is subdued and even disappears… Any ideas?

I wonder if you have a tire that may be out of round. If you have a full sized spare tire, you might try replacing each tire with the spare one at a time and see what happens.

I can assume that all this work also included an alignment. If not, you need to see if the rear axle has been ‘tweaked’ and no longer straight.

Though it is rare, an out of balance flywheel could cause the problem. I’ll still bet on an out of round tire.

If you crank it up to 60+mph, does it matter whether you are accelerating or not? I.e. if you get it up past 60 & then completely come off the gas to a coast, does the vibration change? If acceleration matters think about the inner CV joints.

When the tires were replaced were the old ones inspected for any kinds of odd wear patterns?

“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 and all 4 tires which have been balanced and checked several times. All of these parts had been worn and in need of replacement, the vehicle has 170K.”

Seems like a lot for a vehicle with 170K miles. Are all Nissans like this?

A tire vibration would be there all the time at between 50 and 70 mph. It would be unaffected by weight in the rear.

No, tires is not the problem. I’m kind of going with CV joints, but the weight thing is a puzzle.