2001 Ford Ranger 4.0L SOHC engine problem? (mechanics only, please)?

bkg

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My mother drives a 2001 Ford Ranger with a 4.0L SOHC V6. Whenever you step on the gas (while parked), then engine will rev to about 3,000 rpms and start to sputter like it has a governor or rev limiter installed. I have replaced the spark plugs, wires, air filter, fuel filter, and even cleaned to MAF sensor with MAF sensor cleaner. I checked the computer codes with an OBD-II scanner, and it came back with codes for lean on banks 1 & 2 and multiple cylinder misfires. Any ideas on what's wrong?
It drives OK but not great, and doesn't have the acceleration it should.
 
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