[Diesel Dually]

I was pulling a heavy trailer up a steep hill yesterday when the truck suddenly lost power and started to make a loud noise, almost like boost air or exhaust leak. My RPM's were at about 2200 when this occured. I pulled over and let off the throttle and noticed a puff of brown smoke from the exhaust. I don't know what my EGT was at the time...but I have towed heavier up worse hills with no problems and I don't just hold it to the floor, I am willing to get the tranny in a gear that will not work it any harder than needed, even if it means crawling up the hill. After that the truck will run fine as long as I don't go more that about 1/2 throttle. When I give it any more go pedal the noise returns, power drops and it starts rolling out the black smoke. I had to park the trailer and go get my '05 which towed the trailer with no problem. Any ideas!!? Thanks in advance...