I have a 2003 3500 5.9 Cummins Turbo Diesel "C" with an automatic trans. There is an O/D off button at the end of the shifter.
What differences are there between 2003 and 2005/6 auto transmissions? Is the tranny mechanically the same with different computers? Can the the later model computers be put on the 2003 to give it "tow mode"?

 

it depends on what the rateing is on your engine that depends on what tranny you have that year. if you have a standard output engine then you probably have the 47RE, if you have the high output then you have the 48RE. there are a few internal differences between the trannies, and the 48RE can hold a heck of a lot more power.

mechanicaly the 03 48RE is the same as the following years, however in 05 there was a little programming change with the tow haul mode, then in 06 there was another change to the tow haul as well as the auto accepting an e-brake.

I'm not very familiar with the operation of the tow haul since I have a 6spd.