My bed isn't level from side to side.
I measured the frame to ground distance...not the same...apparently one of the springs is tired. I looked everything over & couldn't spot anything out of kilter.
My local spring shop told me that they were too old to rearch...they said they could add a leaf but that wasn't the way to fix it & it would be trial & error.
Sounded to me that new springs would be the easiest fix...
My pickup is a 1951.