Cause of failure is clear... Broken fifth gear. Looks like a manufacturing defect in the gear. Surprised it ran as long as it did. Lost most of about 7 teeth on fifth and the associated gear. One tooth on fifth had a defect that has been there a long time. Transmission was 12,000/10 months (warranty is 10,000/6 months) down the road from a Standard Transmission rebuild. The offer from Standard to repair was reasonable but, I chose to go with Enterprise instead because they could help me faster being closer. I have only towed 350 miles (last 350 before failure) on the transmission and have not run the truck hard at all. Fifth has been noisy since the rebuild, but all transmissions are slightly different and it was a swap for the one I had.
Oil has been analyzed for viscosity (bulk Castrol from Standard Transmission) and has (under the D445-3 test) a 40 deg C viscosity of 71. 55 Centistokes and (under the D445-5 test) at 100 deg C viscosity of 12. 18 Centistokes. I don't remember for sure but these sound to be right for the Syntorq oil. It is definitely not the 6 speed oil, which was a real concern for me.
Thanx for the suggestions from the members who answered up.
I have learned that it is hard to get a NVG4500 fast while on the road, particularly a full spline one.
With any luck will be on the road by Friday.