If you wanted a softer ride, you would have to switch out the wheels. I'd say do 16 or 17" wheels and adjust the tires to fit. Discount Tire could make some suggestions on what you could go with. Basically you would be filling up more space with tire than wheel, making the ride not so stiff.

If you are running on the michelin tires, go with the continentals. They are much softer.

I still think it is funny you think this ride is stiff. Go drive an SRT 4 or any other "sports type car". Those are stiff. This is very mild.