My car has a droning vibration that starts around 65 mph. At 55 it's fine and as I approach 70 it gets worse. The vibration oscillates on regular intervals, unless I put the car in neutral, and then it's a constant (and worse) vibration until I put it back in drive.

Over the past 8 years I've replaced the universal joints, transmission, brakes, wheels, and I've had the tires "computer spin balanced" multiple times. The vibration has not been affected by any of those changes.

How do you diagnose something like this?

I think I'll get the driveshaft balanced. What would you do to troubleshoot the rear axles and differential?

Any other ideas?

Lee Entrenger (Mushroombert) didn't think this vibration was all that bad when he drove my car last summer. But I sure was jealous when I drove his Challenger and it was smooth as glass at speed.