Update: We completed the trip from eastern CO to New Jersey about a week ago. The temps were mild, less than 75 degrees. The truck performed very strong through the mountains of PA which is where the problem erupted last spring, but at higher temps, 90-93. It shifted into 4th far fewer times than previously, and when it did, the climbs at 65mph were comparatively effortless. In the spring, each time it shifted into 4th, there was not much throw left on the throttle, and within a few seconds, limp mode and power loss. There was plenty left this trip. It seems the replacement fprv is doing what I need. I will not be 100% confident until it is tested in high temps next spring, but I am pleased at this point.
I believe the old fprv weakened quickly with each limp, and had reached the point where it was never "closing" all the way. I think it explains everything, especially since the balance rates on the injectors checked out fine each time observed by "trusted" sources. High temps will be the last test.