my thought was, that 5 years of weight being distibuted to one side of the truck more then the other is what made it sag on the driver side
Could be BOTH the compressing/decompressing of the body mounts from engine pulling on the body/frame the way it does in acceleration and the driver weight. Isn't the fuel tank supported by the frame? Wouldn't be the tank then.
I replaced my suspension and the body still sat higher on the passenger side so it would seem logical that it's the body mounts that are more worn on the driver side.
BOSS