Backfiring in the muffler/tail pipe is sometimes a result of unburnt gas making it's way to a hot spot in the pipes and igniting there. (sometimes)
If you are changing gears to slow the vehicle...1/4 ton vehicles have enough brakes to do the job...braking with the gears is not required.
But you could have a leak in the manifold or pipe that is sucking air in there too.
Big JIm <img src="/forums/images/graemlins/pfft.gif" alt="" />

professional bovine relocation specialist