The symptom is that I can be running the rear tank for a week and during that time I can never get the truck to start on the front tank, and out of nowhere it will not start on the rear but it will now start on the front.

The check valves were an issue in the past so although I left them in place I bought the aftermarket Dorman valves and installed, and those initial issues went away, and seemed to be fine. This seems the same but different.
Pumps are 2- 6 years old. It has new FPR, and the injectors have been pulled, checked and cleaned.

Could this be a check valve issue again? Could it be the old check valves that I left in place? What would be a logical (if any) place to check first?

 

I would probably look at the tank selector switch if it hasn't been replaced.