Most times it's the rear transverse spring that causes the lean. And comes from a weak side to a spring or slightly bent cross member the spring attaches to. Probably comes from an overload and a few bottom outs.
Two ways to fix this. New spring and straighten cross member or a wedge, driven under the low side of spring mount. Loosen bolts drive wedge in and tighten. This is how we used to put wedge in them things for oval dirt track racing back in the day. A little bit goes a long way.