I had the same problem on my 380 I had the wheels balanced last week, and the vibration is a lot less, when driving past 60 mph. The car however does need an alignment so that might be a contributing factor. I have one ball joint with a worn out boot, but a fellow forum member doubts that to be the problem. Here is what he told me, so I'll have mine checked out soon.

"I doubt its your ball joints but if the boots are gone they definitely
should be replaced. Most vibrations are from tires irregularities or
balance but your car does have a common condition that occurs when the
rear strut mount (we call it the caster joint) becomes weak. It allows
the wheel to move forward and backward. At road speed this can cause a
vibration as the movement hits a resonant frequency."

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