I have an 02 F-150 with the 4.2 V6. Under moderate acceleration, the engine will hesitate and miss. Often it will pop (backfire) while this occurs. Auto Zone checked and gave me the P0171 and P0174 codes, meaning lean on both banks.
I took it to the Ford dealership who told me that my 0-rings needed replacing on my intake manifold and wanted to charge me almost $800.
I took my truck to an independent mechanic who checked the motor for leaks around the intake manifold and found none.
I've replaced plugs and fuel filter with no luck.
Any ideas of what could be the problem and how to check for it?

 

I'd recommend getting that fixed with the intake manifold...

 

Lean codes are symptomatic of vacuum leaks somewhere, check the PCV system. If you have around 100k on it, you may need plug wires and maybe a coil pack.