I'd leave the tailgate up. I've heard about tests that show that with the tailgate up the back pressure kind of forms a "ghost toneau" and with it down you lose that. check for gasoline myths and you'll find it. Also, be sure to change the fluids, but for a truck that old I wouldn't go with Synthetic. I hear that if you change over you can actually do more harm than good. The oil sludge ends up forming seals that the synthetic washes away and causes the engine to leak. A good product for higher mileage engines is called "Restore" it's sold at Wal-Mart and Advance Auto Parts here in Alabama. I used it in my old Ranger and loved it, always seemed to help the engine. You can try to run some cleaners through the gasoline system, and be sure to use a high quality (not octane) gas. Try Chevron, I've always had good results with that gas. Good luck!