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The auto used behind the 7.3 PSD was the 4R100 which is a 4 speed. I don't know all of the differences between the two.
I know the 6.0 PSD with the torqshift has a tow/haul mode which I think is a nice feature that I have used when I've left the scales at 22,000-24,000 lbs. The 7.3 with the 4R100 only had an option to turn the OD off.
Its a 5 speed auto, with tow/haul mode that makes it easier to tow and stop. It will sort have a "compression" type brakeing in this mode with makes it easier to stop.
Actually its a 6 speed auto but that another story.
I would suggest you go to the 6.0 forum and ask some questions about a used '03 6.0 before you buy. Bring the VIN # with ya.
... the best thing about the Torqshift was that it was a clean-sheet redesign. (Well, OK, they use the same pan on the bottom...) Unlike the E4OD which became the 4R100 by changing a few things, almost none of which did anything to help the usability or durability, the Torqshift was a new design that significantly improved the product.