I'm gonna throw something out that is a little different. (I have an 04 2500).
The 2005 2500 won't be able to lock out 4th gear (TH locks out only 5th), but the 06 gives you the option to lock out 5th with TH, and also press the button again and lock out 4th, allowing you to stay in 1:1. I WISH I could do that. I'm towing about 7000 lbs, and the truck wants to shift between 4th and 3rd quite a bit. An update to the TCM helps, but it can be annoying. Doesn't hurt the truck at all.
Also, I think the 06 1500 has MDS to shutdown 4 cyl (when not towing). The 2500's do not.
That being said, when I upgraded from a chevy half ton to this 2500 - huge difference. I will never tow with a half ton again. So, it really matters what is important. Payload was a big deal, and now I don't have to worry. But, cruising on the flat gets me shifting a bit.
