I am working on my brothers Expy w/the same 5.4 4x4 setup as my truck. The infamous check engine light came on just as the engine started running roughly at idle. I mean rough as a toothless, armwrestling bar maid. We pulled the OBDII codes and they were:

P1131 - Lack of Ho2s11 sensor indicates lean
P1151 - Lack of Ho2s21 sensor indicates lean

These are both Heated Oxygen Sensor codes and I replaced all 4 o2 sensors and replaced the fuel filter, spark plugs and gave everything a good cleaning like MAF, DPFE Sensor lines, fuel injectors and what not. I inspected everything and the "Check Engine" light is still on.

Any REAL mechanics out there that can help?

 

These codes are indicating that both banks are in a lean condition.

There is probably unmetered air coming into the engine. Check for vacuum leaks - there's most likely one there somewhere.

 

Ditto on the vacuum leaks. Look closely at the PCV system hoses. Have seen several of these lately. They can be hard to follow due to there location.