I learned on my '68 'Camino: If the body bushings are rotted, NOTHING will make the doors/fenders fit properly.
I suspect that things are not so dire on the GMT400 vehicles; there's fewer body/frame bushings, the frame is stronger, and the body (cab) is much shorter. However, I'd be looking at the bushings to see if they--or the steel mounts they attach to--have croaked. That could put undue stress on the cab, leading to poorly-fitting doors.
GM and all the others figured out years ago that if everything is welded together (properly) at the factory, not only do they save time bolting brackets to the body, but there's no time wasted "adjusting" things. Whether it's serviceable later is not a priority; hopefully the vehicle is off-warranty before problems crop up. Thus the move towards non-adjustable doors, non-adjustable front-end alignment, etc.
I suspect that things are not so dire on the GMT400 vehicles; there's fewer body/frame bushings, the frame is stronger, and the body (cab) is much shorter. However, I'd be looking at the bushings to see if they--or the steel mounts they attach to--have croaked. That could put undue stress on the cab, leading to poorly-fitting doors.
GM and all the others figured out years ago that if everything is welded together (properly) at the factory, not only do they save time bolting brackets to the body, but there's no time wasted "adjusting" things. Whether it's serviceable later is not a priority; hopefully the vehicle is off-warranty before problems crop up. Thus the move towards non-adjustable doors, non-adjustable front-end alignment, etc.