It seems obvious but I’m gonna ask anyway - is there a strong gas smell that would indicate a leak?

Under the truck, on the driver side near the transmission, is where the fuel lines cross from the engine to the frame. They both have a section of rubber hose to allow for flexing. The smaller of the 2 lines is the return line to the tank.

Pinch off the return line using hose pinching pliers if you have them. If not, a pair of vice grips will work but you don’t want to clamp the line too tight and if the edges of the jaws are sharp, I’d recommend wrapping a rag or a couple of paper towels around the line first.

Check fuel pressure at key on - if it holds steady for more than 2 minutes, remove the clamp from the return line - if it drops to zero immediately, the fuel pressure regulator is bad. You’re gonna have to access the injector assembly to replace it because it’s attached to the body under the intake. And I’m gonna suggest if you haven’t already replaced the injector assembly, go ahead and do it while you’ve got it apart.

If the pressure doesn’t hold at key on while the clamp is still on the return line, the problem is isolated to the injectors (very common failure) or the fuel pump.