Bad heat treat on one? Pull them both out, if the lengths are significantly different side by side unloaded, the short one is probably fatigued and has taken a set. It happens, I bought a new set of Rancho springs for an old jeep, front and back, every time I hit a decent bump, the right front spring lost a little more arch on the main leaf, and I had to replace it, no, Rancho did not stand by their product, but that's another story. Spring rates are repeatable and standardized by their carbon content and the respective heat treat process, another factory spring will be close enough for safeties sake. Buying a matched set only ensures the same amount of flex cycles and usually both will have shared in the same heat treat batch. Ergo matched springs. But by all means, ascertain you have a bad one, before you start spending money.
