I also have a 2003 3500 dually....Like clutch said the early ones have the 47re. If it has the 47re it will have the standard output motor, if it has the 48re it can have the standard or the high output motor.
I have owned the 47re and the 48re equiped trucks and I can say I like the 48re much better. It shifts without hesitation or being sluggish, it also holds overdrive better than the 47re. It shifts into OD at 80 kph. religously unless it is under a load.
The 03 2wd. is equiped with rack and pinion steering and is sheer pleasure to drive, on the highway it is effortless (no wandering)
These trucks do a little better in the mileage dept. than the new ones also. Pay attention to drive axle ratios...they came with 3.73 and 4.10 ratios. Unless your really pulling heavy the 3.73 is the one to get. I think it's rated at 13,500lbs. towing capacity. The only weakness is the lift pumps, the 03's pump is outside the fuel tank and has to draw fuel from the tank making it work harder than the 04's where the pump is in the tank (on most) and pushes the fuel out. This is easier in the pump.
I am really enjoying this truck and would recomend it to anyboby.
Good luck with your purchase.