I will wash all the mud off and give this a shot. But it seems like the smell is coming from the engine not the rear.you could inspect the tank, checking for wet spots. these trucks have sh!tty fuel tanks, all are prone to rust thru in places contacting the skid plate/tank straps. ask me how I know....
It's a 89I had a fuel smell problem too. It was my fuel inlet valve's o-ring on my primary side. 5 minute fix for me. ... what year is yours? carb or EFI?