First question: are the trucks the 5.4 v8 or the 4.6? If the 04 is a 5.4, have the spark plugs been changed? If not, no deal on the 04. (That spark plug change will run you between 500-1000+ depending how many plugs snap off in the head, google it.)
So, an 04 with a known transmission problem and the baggage of dealing with your father vs an 02 in sound mechanical condition and an 8 foot box? I'd vote for the 02. Plus you'll save some money up front which can be put towards other equiment or your insurance.