[James]

Backfire through the carb when you shut the engine down! That is strange. I don't believe the transmission could cause this.

Having the idle too high might make it diesel when you turn the engine off and causing the engine to kickback (or puff) through the carb.

Other thought-
Does your truck have a "Gen" light on the panel? I have seen in my distance past this was caused by an engine idling to high and the gen light circuit (light would be glowing with the key off) back feeding the electrical system with enough power to make the engine barely run. You can try removing the bulb and see if the problem goes away.

That my thought on it.