Non-Mopar: NEED HELP- 1995 Ford F-250 Diesel

Thank you all for viewing. My cousin has a 1995 Ford F-250 pickup with the 7.3L Powerstroke Diesel motor. He bought it used, and it was previously used/maintained as a fleet vehicle.

He was driving home Monday and the truck started to stumble and eventually died. He changed the fuel filter on the side of the road and it started back up again but died about a minute later. He could get it to run, but only above ~1,500-1,700 RPM. It would not idle at all.

He got it towed home and we have since checked the following:
-All terminals on the existing PCM were checked for proper resistance, and another PCM was plugged in from a running truck (same year, make, model) and it still won't idle.
-The fuel pressure after the lift pump was measured and verified (holding at ~70psi).
-The fuel is indeed flowing out of the lift pump.

The truck won't run now. He can get it to run in his driveway, but again, only when he holds it above ~1,500 RPM.

Has anybody owned/maintained one of these and run into a similar problem? I want to say it's a dying injector pump, but I'd hate to see my cousin spend $500-$800 and have that not be the culprit.

Any and all help greatly appreciated. Thank you all again.
(and sorry it's not a Mopar. If it was, it would probably be running :tongue3: )

-Mike