Express/Savanna 3500

I think the ride is pretty bad stock IMO. Mostly the rear. It's really under-dampened. Its not bad empty, but gets bad when fully loaded. Bad enough I had to stop renting them for group events (they got nicknamed "puke vans"). Good monotube shocks like Bilstein or KYB should mostly fix that. If you get close to the RAWR, you might want some spring help too, to keep it off the bump stops. For that I would be inclined to look at Sumo Springs, since they won't add harshness and won't ever need maintenance like air bags.