Front Suspension Lower Ball Joint problems of the 2001 Cadillac Seville

I bought a Cadillac Seville from a used car dealer, it vibrate when I drive it. The whole car vibrates while driving it on a smooth road. The car dealer replace the tires, tie-rode, lower ball joints, and steering arm or something to that effect and done an alignment on all the wheels. It still is vibrating to this day. They say they can 't duplicate the vibration, and don't want to ride with me to show me what they are considering normal and I can show them what I am feel as abnormal. They even took it to at the Cadillac dealer and they done some expensive machine balancing on the tires and it still does it. One mechanic said that I will have to wait until it gets worse to find out what it is or to get it fixed. That does not sound good to me at all. I don't know if there has been any injures or death related to this problem, however I don't want to the first.