Ball joints are a red herring. They don't explain this sort of vibration. Usually, they just let go without warning. If there's vibration at all, it will be random and at lower speeds.

A vibration around 60 mph is nearly always wheel or drivetrain related. Did you say this is a 190DT? If so, check the space between the front swaybar and the oil pan. there should be at least 1/4 inch of clear air. If the engine mounts are bad, the engine will settle down onto the sway bar, and you will have not-good vibrations. It seems to be a problem unique to 190D 2.5's.