Ford: 2004 F-150 5.4 auto 2wd lean codes both banks, vacuum leak, vacuum line

Question
I have a 04 p/u that recently back fired through the intake. I was leaving on vacation so instead of trying to fix myself, I took it to the local Ford Dealer thinking the professionals would find it quick and be on my way. I figured it was probably a vacuum leak. I was informed that the maf was causing problems and needed replaced and did so. Well no change. They told me that they check the vacuum line for the p/b booster and it is ok. I removed the throttle body and spacer plate last night to check those gaskets and they were fine. Any ideas were they are prone to loosing gaskets or blow out intake seams or material.

Answer
the intake phylum on that year is made from high impact ABS-AKA, Plastic. they are known for resistance to corrosion however. best guess with out looking at it myself, is the intake gasket between the upper and lower phylum. please restate your question if this does not help.