Here's a few thoughts from things that I've learned over the years...
You can check the electrical side in the truck but the oil/fuel side is hard to check until the inj is on the flowbench.
-Verify good electrical to inj and solenoid ; solenoid pulls about 3.2 ohms resistance
-Pull the solenoid top off inj and check the armature plate for looseness and clearance. Must be tight and will have .002" or so clearance. The plate screw must be tight as it moves the poppet (shuttle) valve when the solenoid turns on/off. If the armature is loose, this will disable the inj even though it gets power. Tighten screw or replace the screw with a new torx head replacement. these take only a few in/lbs of torque. take care with removing the solenoid.
-Turn off an injector and watch it while the motor is running. If you see a little oil leak out the spout, the poppet valve leaks. ( This is hard to see.) If you see oil coming out the base of the inj where it enters the head, the oil oring is leaking-- both may show low ICP
Down below; (these require inj removal and rebuild/replacement)
-Nozzle springs break in old injs and gives poor fuel delivery/spray
-Barrel /plungers score with poor quality fuel and age
-Internal orings wear out
-do a compression test. Old valves leak when cold and seal better when hot