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It sounds like your Intake Manifold Gaskets are to blame. There have been many problems with the gaskets along with the bolts the hold the lower half manifold on. It is a very common problem with the 4.2L V6 F150.
Ok then. More than likely the lean codes are coming from a vacuum leak somewhere on the motor. The PCV hose is another big concern and could lead to a vacuum leak. I am sure you have already done it, but inspect all vacuum lines going to the intake manifold. Also the lean codes can be coming from low fuel pressure, clogged fuel injector or a clogged fuel filter. You may want to check the fuel filter and possibly check the fuel pressure if you can.