I've got a 96 deville and it also has a rather stiff ride. I recently rode in a newer model and you can't even feel the ground under you. That's what I would expect of a caddy. But in my car, I can feel every bump in the road. I am sure that the struts probably are stale and no longer absorb the shocks, but OEM struts are like $600 each. It's an expensive proposition to restore the cloud-like ride. I think your car has the same problem though. Since most of the shock is absorbed in the front, I would think you'd need to replace the struts up front.
I know that there are companies like Strutmaster and Bostonstruts that make suitable "passive" replacements, but I'd like to have original equipment (quality).