The old saying is "go down in at least as low a gear as you went up in".........
The Allison 5 speed auto transmission behind the Duramax, when in tow/haul mode will downshift as needed to easily decend a grade undr full control. Just tap the brake to over ride it and it will manually downshift. I believe that the new Ford 5 sp auto also has a similiar feature. In my case, I see no need for an exhaust brake.
Have a GREAT day !