1967 C10 - Rear frame damage or rear springs?

[mr.precision]

The rear of my 1967 short bed step side sits about 2" higher on the passenger side than it does on the driver side, when measured at the wheel well. It's also quite visible when viewing the bed from the rear.
Not too many frame people around these parts, so before I go to a shop that can measure the frame my question is: do the springs wear in a different way over the 51 years this truck has been used? Or is this definitely a bent frame that will require some attention.
I don't know where to start, but it's gotta be fixed.
Thanks