Thinking about this again,
at those conditions of 1400+ rpm and low simulated pickup mph
It is not likely that the EGR has been turned on
since the exhaust back pressure would be low and less than 3 psi.

Much more likely that two or more cylinders are running lean.

If the truck is now idling more smoothly now that you fixed one vacuum leak, that is another clue that the air to fuel mixture in a few cylinders has gone from very lean down to still lean.