I would be interested in what the pressure of main line is doing when the trans will not move in OD after the shifter is placed there from park.
Pressure is boosted when t he shifter is in manual 1 gear.
What is the history of the trans in the 2000 truck? Mileage? Recent repair or rebuild? Sounds like you've had the trans out and apart a couple of times
A pump with excessive clearance can actually stop pumping when a load is applied like shifting from park into gear.
There may also be a problem with the forward sprag assembly. I had a customer that claimed our sprag parts were bad that he had just purchased. The problem was he had put a new sprag against worn out sprag races and took out the new sprag.
Dont worry how old you are... it is the mileage on your body that adds up.