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Hello and thank you for your question,One very common caused for these to fault codes to set can be caused by EGR passages in the intake manifold becoming restricted. In order to determine if in fact that is the case you will need to remove the upper intake manifold to expose the runners of the lower intake and clean the carbon from all passages . Its very common for them to restrict on the 4.2 as well and I would suspect thats what you dealing with here. If the engine is actually missing you can also disconnect the vacuum line from the EGR valve and see misfire clears. If so then the passages are restricted.
If you end up having to remove the upper plenum to clean the passages it would also be a good idea to install the updated o-ring gaskets between the upper plenum and the lower intake manifold, along with the updated isolator bolts that hold the upper plenum to the lower intake manifold. There are know issues with the old style gasket and isolator bolt leaking .