Since the OEM have an L and R spring, I figured the after market should as well, since the springs have to carry a different amount of weight.

When I talked to National Springs a few years back, they wanted the overall weight of the rig, front/rear and left/right weight distribution, to build me a set of springs that would carry the weight, flex on the trail, keep a nice ride, and would lift the rear 4". They also wanted about $600 for the set, which is why I still don't have them(built my own for the meantime). I figured that wanting the various weight distribution was an industry standard, so you would get a properely built spring.