What’s causing the vibration?

[ramrat]

Car 92 240sx convert. Things I've replaced because I've needed to:

New - KYB shocks, tires, ball joints (front), tire rods, tension rod, alignment.

Problem is before and still after the above mentioned items going down the road between 55 and 65 mph I get a strong vibration that feels like its going to shake the car to pieces some times while other times you can feel it there but its not to bad.

Could this vibration be the cause of bad motor or transmission mounts? The Convertible has 150k miles on it.