Sorry dude. I gotta call you on this one. My ECM did not have the software in it and it needed the re-flash to get it. I was standing there when he did it. I was also the one to point out that the function must be enabled using the DRB. It's under option 3 (pretty sure) of the engine menu. It's listed under the misc menu.
As for the tire size thingy, I have to call you wrong again. My dealership tech and I, sat at my truck for an hour with one of their DRB-III scan tools. You are correct that there are limited sizes to pick from. Mine only allowed two sizes to pick from. 245/65 R-16 and 245/65 R-16 (don't know why) and no others. However, if you select an "other" Dana axle option, you can program the PCM to allow any tire size. But then you run into another problem, your tire must turn more than 644 (I think) Rev's per mile. If not, then you must change the number of teeth the PCM thinks the ring gear has. Hence, if the ring gear has 200 teeth (couldn't find the actual number anywhere) but you tell the PCM it has 180 teeth, you will see a 10% increase in speed. I think the stock ring has 120 teeth, but we couldn't find it anywhere. So we were left to guess when we re-programmed my truck.
Nothing personal towards you, but this is what we (I) did on my truck.