1999 Ford F-150 OBD2 Codes P0171 and P0174: Diagnosis and Fixes

I'm experiencing OBD2 diagnostic codes P0171 and P0174 on my 1999 Ford F-150. These are engine lean mixture codes, indicating the air-fuel ratio is too lean in both the left and right fuel banks. I was advised to replace the mass air flow (MAF) sensor, which I did — but the codes persist. What should be my next step? The vehicle is a 1999 Ford F-150 with a 4.6L V8 engine. Note: P0171 and P0174 indicate that the powertrain control module (PCM) is compensating by adding more fuel to maintain combustion, which suggests an underlying issue such as vacuum leaks or low fuel pressure.