As I mentioned as one of the possibilities is something missing/removed. Do you have the FACTORY vertical stiffening bars on inside back of cab? Attached to an under-the-rear-window horizontal piece and attached at the bottom near floor. There are one, two or three of these. They may also be attached to the cab back. This would prevent oilcan.
Big flat areas can be warped by patching and welding.
Stripping a cab could, possibly, maybe remove some structure that was stiffening the cab back.
Oil canning doesn't happen out of the blue.
Maybe somehow it will tighten up after reassembly. If it's not noticeable or making noise going down the road, does it need to be fixed?
Good luck.
