If the truck is sagging with just a shell on the back, it's time to address the springs.
With just the shell and an empty bed it sits within an inch or so of level.
But with the addition of maybe 300 lbs (stall mat "bedliner", extraction gear, hunting gear, and a cooler or two of deer meat)and it's sitting well back on the rear springs.
It handles fine as is, so if I can level it out with a typical load (using either option) would I be ok mechanically?
I know I could take off the leveling kit, but if the rear springs are shot (like RC says) it would still sag with a load. And I like the option of going with slightly taller tires when these wear out.