This is quite common. We get this call at our shop all the time. Since its a 04 without all the exhaust regen stuff, I would just siphon the fuel out. We have an electric siphon pump that we run the hose down the filler neck, most trucks we can get it to the fuel some have baffles and you end up pulling the filler neck hose at the tank and siphoning from there. If your real lucky theres a drain on the tank but probably not. Then just drain all the gas / diesel mix you can possibly get out. Refill with diesel and your good. What little gas will be left will be no problem on the older trucks like this one. Now if it was a newer truck we would drop the tank and get all the fuel out. Hope that helps you.