Monitoring O2 activity pre/post will give you an indication of converter condition as well.
Yes there is a back pressure gauge that installs in place of the pre-cat O2 sensor to check for it being clogged but not for converter inefficency.

If the MAF sensor was bad, wouldn't we get some code?

Besides "speed signal missing" (c0226 and c0222), and a pair of potential electrical problems (c0281, stop lamp switch, and c0252 electonic brake controle module), everything seems OK.

Also, if it was a MAF sensor, why would it only affect it at idle?

Does the truck ever have an issue starting (turns over longer sometimes)?

It starts like a dream

No, a failing sensor will not always set a code.
As one of our instuctors said, no signal is better than a bad signal, coming from a sensor.

Seen MAF sensors idle good but fault out at highway RPMs.

I am actually a fan of the MAF sensor cleaning.

I've discovered that it has a K&N filter, which can cause a dirty MAF sensor if improperly used.

The idea of K&N filter doesn't appeal to me, even after reading all the "rave" reviews.

Ran 3 different Fuel Injector cleaners on 3 different tanks
Chevron Techron
Gumout Rogain
Redline SI

Cleaned the MAF sensor before the 3rd tank.. it was very dirty.

MPG, which on the first tank was under 10, on 2nd under 13, and on 3rd over 14, is probably going to be as good as it can get

Took the 4th emissions test, after a pretty long drive (so engine was warmed up).. while the numbers seem to have improved, still not passing @ idle.

What should I attack next. (already replaced wires, plugs, 02 sensors)