Since you have checked all of the usual things and the rubber eyelet bushings are still good and you decide to remove and swap the leaf packs side to side; I would place the leaf packs on the ground with the eyelets on the ground and measure the distance between the ground and the high point of the leaf arc.
If there is a noticeable difference in the distance from the ground to the top of the arc on both leaf packs unloaded; there is a problem with one or more of the individual leafs. This problem would be more noticeable when loaded....
Not sure how old the packs are, but you if you are the original owner you might consider contacting Rubicon Express Customer Service to see if they will warrantee them.....