The 2004 and earlier Fords had a light duty weight range, so a 11,500 pound GVWR required DRW back in 2004, while in 2005 the larger 18" rims and 3,400 pound rated tires allowed you to carry a 4,000 pound load with a F-350 crewcab.
Howver as stated above the 2003 6 liter diesel was not the best design, and the 6.4L was mildly better than that one. Problem was that the engine oil cooler was inline with the exhaust gas cooler. When the coolant plugged the first cooler, then the second one would overheat with 1,100F exhaust gas, then leak water into the air intake. Installing a coolant water filter solved the problem, but never became a popular option among Ford trucks. More popular was the $12,000 to $15,000 engine rebuild. The $18 filter twice a year seems more economical to me. . . .
Fred.