If it's sagging then something isn't the same left to right. Obviously. Have you done some measuring while on a flat surface? Could the cab mounts or the body be to blame? Are the springs arched the same in the rear? Has the suspension taken some damage somewhere along the line? Set up a laser level and use that to create some reference points. I don't think that changing to a different suspension is the answer. But correcting what you already have is. Some investigation is in order.