"I have tried idling the truck in Neutral (since I know the tranny pump doesn't circulate fluid in Park)"

Is this just on the tranny behind the 3.9 or the 45RFE also? I never understood why some tranny's don't pump in Park.

 

Not sure which tranny's do or don't pump in park... for 40 years, I've been told to check the fluid while in neutral. Maybe not necessary in some of the vehicles I've owned, but it's always worked.

Good luck!