what is the black sheild in front of the muffler on the stock exhaust? if i change my exhaust do i replace this piece?

 

that is a skid plate and if you change the muffler/exhaust you will not need it.

 

hmm i think most people here think it is a way to direct airflow from behind the rear bumper. anyone care to elaborate on what this is?

 

that sounds reasonable, but it is one hefty piece of metal. Mine was so beefy and the muffler hangs so low I assumed it was to protect it. My borla system tucks very nicely up under the bumber and is pretty much out of the way of wind, drive ways etc.