[twindman]

I was worried when using tow/haul on a down grade and I applied the brakes. I was near red-line on the engine and it downshifted. This put it way over red-line; somewhere around 3800 or so. My (2004) Silverado 2500 has a red-line at around 3100-3200. I have noticed other diesel engine years (I think Silverado) had much different red-line limits.
Finally to the point! The mechanics at Chevy weren't too sure about red-line while slowing, but I think on DieselPlace.com someone said exceeding red-line while using it for slowing was okay.

I also found (and read somewhere) that when you apply the brakes for 3(?) seconds it will downshift. So I play around a bit and when going down a long downhill and get near red-line, I hold the brakes until it downshifts. Then if it still gets too fast for me, I will hold the brake on for around 2 seconds to slow a bit, then release them. Repeat if necessary.
I am not sure if all years of Duramx are this way, but that is my story.