I also have the p0171 in a 5.4!
I'm at 198***mi with the 5.4l on a 2002. I've had this code for the last 2 years. Over the 2y I've changed O2, maf, redo all vacuum and fuel hoses, EGR and selenoid, purge valve, coils, plugs, pcv, iac, fuel pump, throttle body gasket, tested cats. My trims are off the charts -30,+10 ... At this point, I'm suspecting an exhaust leak. The van has enough power to move around.
Sometimes when is cold I hear this humming sound, I traced it back to one of the cats, it might have a hole somewhere or maybe is up the line, you know a broken stud....