I will make a wild guess and say that it has a Ford chassis. What you are feeling is side to side roll, or waller. Early Ford F53 chassis have very soft spring rates that allow side to side roll. It may not have a rear Pan Hard bar/Track Bar either, which allows the rear axle to oscillate under the chassis in a circle. If it does have a Ford chassis, the first thing to do is check if it has a rear track bar. If not, installing one will make a big difference. Moving the sway bar links to the inner hole will give greater resistance to roll also, and does not cost anything. After these mods, adding either aux. air bags, or Timbrens to increase the spring rates. When we first got ours, I though it would turn over in slow turns, as it would lean over until it hit the bump-stops.