Have you replaced the o ring seals at the fuel heater ? you could have been getting air in system and caused cavitation damage to the pump it should go to fuel shop and be torn down and inspected seal kits are cheep and labor isn't bad If you do that you will know where you stand !

no i have not done anything with the fuel system on this truck ever, other then changing the filter of course. Where is the fuel heater located?

wouldnt it make sens to say the lift pump is of concern, being that the pump has seized in the past, of course it only does it when im out of diesel but wouldnt it make sense if the injection pump is only recieving just barely enough from the lift pump? my only concern is that the system should still prime up with a bad lift pump... but not really becasue i can prime it up but then after a few sputters its got air again, that could jest be beacuse the lift pump isnt providing any fuel for the injection pump...