Doug,
What you suspect is Sumo spring is actually the stock factory bumpers that come with a Workhorse "W" chassis. They come in different colors depending on the axle weight. IN addition to your front end alignment as recommended, you may want to consider a thrust angle alignment that insures that your rear axle is in perfect alignment with the front axle. I am sure that you have seen vehicles going down the road looking like they are "crabbing". A front end only alignment will not correct this. The problem is that most truck alignment shops cannot do this so you will have to search for one that has the proper equipment.