You have a live axle rear suspension. You will always have a harsher ride than IRS suspensions. You can try progressive springs. Eibach and others have them. They will all lower your car about 1.5". If you don't know what these are, they have variable spring rates. For example, one might start at 250lbs on initial compression and work up to 500lbs as the spring is compressed. The softer feel on initial compression will reduce the amount of road imperfections that you will feel.