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On mine I checked all the fluids, changed the oil, oil filter, transmission fluid, transmission filter, flushed the coolant system, changed the spark plug wires and changed the spark plugs. If you haven't done a transmission drain in awhile, I wouldn't even bother.
Don't forget air filter, mine is a K&N filter you have to clean, but saves a few $ over the long haul.
Also slick50 oitl treatment has helped my 3.9 get 180K.
I put a dynomax muffler on my truck and got an extra 20 miles per tank. It will cost more but run prem. gas in it. that will help gain miles per gallon to. Like they said new spark plug wires, plugs, fuel filter, um basicaly everything else that has been said. try adding seafoam to a few tanks of gas to help clean out the fuel system. Other than that i have no idea.