Tire pressure is the biggest adjustment you can make. Running unloaded or loaded lightly the rear tires should be at 40 psi (read your manual, it's there!). If you are running wider than stock tires, you won't be able to do that because the tires will rub and that will cause failure.
The next step is a lightly damped shock. I run Rancho 9000's set on 0 or 1 and they are OK. Turning them up higher makes the rear very stiff. The Bilstiens might be better.