Sounds like a broken shock mount to me as well. Our '95 855 had that happen six months after we got it - It was not rusted, and we did not recall any incidents that would have damaged it.

I could hear the top of the shock hitting the bottom of the car

As for a rear sway bar - our '96 855R had a rear sway bar.

The suspension on that car was terrific - In Michigan, I had to swerve around a motorcycle that was in the center lane of a 3 lane road as I passed a truck at 70 MPH - I couldn't believe that we did not spin. I was in a proper passing lane, the cycle, stopped to make a left turn was not!

While the handling was great on that car, the ride was not - the car would react strongly to ANY bump - I told people that it would bump when I drove over the shadow of a telephone pole! On bumpy Vermont roads i doubt you will be happy for long if you add a rear sway bar.

Despite that, we drove that beautiful car for 11 years - by that time it was 17 years old.