Today, I'm going to change the lift pump as I'm trying to eliminate the straight forward items first:
https://tchavers.wistia.com/medias/a15fr2ob4o
But before I start this, does this appear to be a lift pump issue. See vid.
I opened that valve to check flow and see if I was getting air in the fuel somehow. Because the truck shuts down unexpectedly as seen in the video.
After I will check the tank as I do not have that panel behind the rear seat to check the fuel sender.
Since no one likes to look in the TMs, I did it for you. The fuel system troubleshhoting section is around page 2-95 of TM 9-2320-280-20-2. There is even a section on "how to use the troubleshooting guide".
STEP 2. Does the engine start and stay running?
If no, go to B, Page 2-106.
......
pump pressure needs to be at least 3 psi.
Or, remove the inlet line from the injection pump. You should get 1 pint of fuel in 30 seconds of cranking.
The previous steps tell you what to unhook to keep the engine from starting.
This will test your lift pump.
Clean out a plastic bottle and mark a line for 1 pint. you can put fuel back in the tank.