A little more detail might help, like: "Backfiring" thru the exhaust?? (out the tailpipe, likely to blow a hole in the muffler) OR: Back up thru the carb, thru the air cleaner (aka "coughing" thru the carb) Sorry to sound picky; local usage in different places sometimes confuses the two. When, under what engine conditions( warm? accelerating? decelerating? steady driving?). "The obvious" could mean different things to diferent people, and something could be overlooked. Any recent work on ignition or fuel system BEFORE problem started, or truck having sat a long time, and trouble started when put back into use? Good luck.
