The current automatics don't lock the torque converter in first gear, so an automatic truck with exhaust brake will not work at these low speeds. Of all the trucks I've owned with an exhaust brake (02 Dodge/Cummins 6 speed manual, 07 Dodge/Cummins 6 speed manual, and 09 Dodge/Cummins 6 speed auto), only the manual trans trucks exhaust brakes worked at that low of speed. The automatic when in second gear unlocks the torque converter between 15 and 20 mph when decelerating, which disables the exhaust brake. If the OP had 4 wheel drive, since he was on a dirt road, could have used Low Range 4x4 to help with the descent.
Rich