I remember a thread from a long time ago adressing this very problem. The main theory was that the trucks were secured to the transports of trucks and trains on the left side thus weakening the rear springs. I remember several people replacing these.

Something you might want to try first is to loosen the bolts on the hangers and shackles, retighten and remeasure the distances. If these bolts are torqued to tightness without the full weight of the truck sitting on the suspension, it could result in one side sitting a little higher than the other.