it's worthwhile to shop around for O2 sensor.. i paid somwhere around $40 a couple of years ago.
i'd run the ecu codes and see if anything pops up... garage --> how to
miss could be all kinds of thing.. start with the basic tune-up.. plugs, wires, rotor, etc... heck it could be just a loose boot on a plug..
the rich condition is probably the 02.. that's the only time i recall mine ever running rich.. clean the throttle body too while you're at it. btw. those aren't terrible mpgs at all.. not the best but not completely out of range for these trucks.. that's all i ever get out of mine and i am all tuned up.