Diesel Bombers - Transmission Lockout grumble?

I have a 2002 5.9 cummins in my Dodge 2500
I purchased the truck used with 450,000 km on it.
It was basically in really great running condition.

From the time that I bought it a year ago, the transmission seems to have trouble staying in lockout.
The transmission shifts fine from first to O/D and I don't think I have any complaints about the shift points.
But when it gets to locking out, it seems to be like it's not fully engaging and it kinda grumbles.
If I back off the throttle, it will lock out and generally will stay locked out, depending on the terrain I'm driving. But, if it does come out of lock out while climbing a hill or something, I just have to feather the throttle pedal once regaining my speed to get it to securely lock out again.

I do transmission oil and filter changes every 10k and use Amsoil
I have replaced the speed sensor in the differential in an attempt to solve another problem, (the ABS and Brake light stay on)
And, I have changed the TPS (throttle position sensor) as it was definitely defective.
I have changed the Transmission relay in the under the hood fuse box, I have two new batteries and have gone through all the ground connections under the hood and replaced them with new wire and ends.
I'm not sure if either of these things affect the lockout.

I would love to find that it's just a pressure sensor or something really simple to solve this problem.

Thanks for your thoughts and help.