Hope your not planning to put big tires on the 04 F150, mine got 10mpg. I had an 04 F150 CrewCab 4x4 XLT.
Gas Breakdown @ $3.33 Gal / 10 mpg = $0.33 a mile (what I got)
Diesel Breakdown @ $3.99 Gal / 15 mpg = $0.27 a mile (what I get now)
I'll take the diesel all day, plus I don't have to fill up all the time.
Dude just keep the diesel, unless your keeping stock tires on the F150.