Uneven fuel level in tanks.

[Rawze]

trash in the return line splitter valve above the tranny?

Maybe disconnect the fuel lines at the tank and at the fuel-water separator and blast some air thru them from both directions to ensure they are cleared out?

Has this truck always done this? or did it just start?.. maybe you got some trash in one tank and its clogging the pick-up tube on the one side?

Maybe the vent tube got a wasp (dirt dobber) nest in the end of it?--- This time of year that is common.

also might want to check for a kink in the line?