I have had these codes for one year. I have looked the engine over and over again. All the vacuum hoses look good.
Are there any secrets to finding these leaks? I have a 2000 Expy with the 5.4.

 

Did you check the PCV elbow at the top rear of the engine?

 

I will look again, it's way under the windshield?

 

I will second what alloro said, good chance that your problem.

 

I checked that elbow again, it's still soft and supple, no cracks.
Any other high suspect areas to look at?

 

Under the battery tray there are hoses that get damaged from battery corrosion washing down.

 

I just fought this problem for about 4 months. This weekend I replaced all of the vacuum lines on the top of the engine and the rubber adapter in the right side valve cover. While doing it I removed and cleaned the throttle body and the throttle body spacer (the part between the throttle body and the intake manifold) using the directions posted in this forum. I replaced the EGR valve, the IAC and all of the gaskets. Cost about $200 from dealer for lines and about $100 for EGR and IAC and took about 3 hours including cleaning time. My Expy has 171K miles on it and was way over due. All of the parts were carbon caked. All of the rubber joints on the PVC line were firm, but were loose and rotated on the plastic line very easily. The others showed various levels of heat damage and wear. I did not want to chase this leak forever so went for the nuclear option. Removing the spacer and throttle body made removal and replacement of the lines easier. Check Engine light and whistling sound in the cabin that I had long ago learned to ignore are gone and truck seems to run better. You may not want to go this far, but ask yourself how much is your time worth. Good Luck.