You have three viable choices available to you that will be a great improvement especially on the extended body Vans .
#1 The tried and proven Hellwig Anti Sway system . www.hellwigproducts.com
#2 Airbags , such as Firestone and Hellwig .
#3 Modification of your spring packs or replacement of your Springs www.deaverspring.com
The Hellwigs would be the least expensive and easiest of the three for installation and a very good starting point . I have all of the above and have used on numerous vehicles with great success .