I have a '01 1500 chevy. The transmission gave it up at 80,000 mi. The sun gear spline striped. Trans man said that was typical, he installed a heavier gear with hardened splines and an extra clutch plate. Yours may have the heavy duty gear since it's a 2500 and a bigger engine.
I asked him about the tow/haul button. He said use it to get a very heavy trailer rolling, then I could turn it off... Like I can remember to do that every time!
I never use it. This trans has 170,000 mi., changed the fluid once, still looked and smelled like new when I changed it 100k back.
The few times I used tow/haul all I noticed was bad gas mileage, but then I was pulling a trailer, so no way to really tell.