[BR3W CITY]

Is the engine cammed?

Try doing a CASE relearn. Search here or on LS1tech for the procedure. It puts the cam and crank sensors back into phase with each other, and should clear the code.

The lean condition may just be a side effect from the backfire. When you clear the codes and do the CASE relearn, I'm guessing it will go away on its own. If not, you may have 2 unrelated issues, but don't fret about that until you try the above first.