shaking over 55 mph

Could be any of the above, or a combination of them.
Misaligned tires with bad shocks will make a tire hop, as will bad shocks and an out of balance tire.
Or just a bad shock
Or just out of alignment
Or just an out of balance tire.
Or bad bushings causing a misalignment and an out of balance tire/s

See where this is going? :-)
It's kind of an all or nothing type thing.