THERE IS NOTHING LEAKING!!!!!!!!!!!!!!!!!!!!!!! What you see is the factory applied udercoating wearing off due to age and abrasion from road debris slinging from the tires, which in turn is showing the paint underneath! Remove the plastic liners, scrub and clean the area thoroughly, let dry, scuff with a RED scotch brite pad, re-clean with a wax and grease remover, and then mask and re-coat with RUBBERIZED undercoating (or if you want extra protection apply one of those do-it-yourself truck bed liner coatings instead).