Your right, the Duramax with the Allison in 2500 and 3500 doesn't have an exhaust brake. The have have an intake restriction that does in effect the same thing. In the '06 models it is activated in two different ways. If your not using crusie control, you press on the brake pedal and slow down, if you subsequently speed up with out using the accelerator the transmission will down shift. You can continue this process down to 2nd gear. If you are using cruise control, the transmission will downs shift if your speed exceeds the set speed by 6-8 mph and will continue down shifting until you reach the set speed. During down shifts for braking the rpm will at times exceed 4,500 rpm and make a heck of a noise, but that is all well within spec as decelleration red line is 4,800 rpm.
The company that produces the Mountain directories also has a computer version that contains the same information. I find it quite useful when planning trips through the mountains.
Ron '06 Chevy 3500 Duramax HO Diesel, Allison 6 Speed Automatic Transmission, Crew Cab, Long Bed, Dually '01 Alfa 35' Triple Side 5th Wheel