I'm not sure who you are talking to. My 04 and 05 ford 6.0L have engine braking if you are in Tow/Haul Mode.
You just tap the brake and it downshifts. My cousins have 04 Ram 2500's and his doesn't do that in T/H mode. He was complaining about burning up brakes towing his fiver. I never have issues down hilling it in the rocky mountains. But all newer trucks do have engine breaking on diesel pickups. If you are referring to pre 2004, then it is manufacture specific.
Down the road from work we have a steep hill and I kick my truck into tow haul mode every day to go down that hill. An F350 will scream down that hill if you are not in TH or on the brakes.