i notice the same thing, mainly while i'm in the cab and looking at the box in the rear-view(left-side seems lower). initially i attributed it to my viewing angle with the mirror. looking at the truck from behind, the truck(or at least the box) appears to be leaning slightly left. however, when i measure it(ground to top-of-box) the numbers are even within reason.

one thing i have noticed recently is that my left rear shock is leaking and needs replacement but that wasn't the case before. is it possible that the lean has anything to do with the shock on the left side being mounted behind the rear axle versus in front on the right side?

anyone have recommendations for replacing the shocks? OEM? TRD? aftermarket?