I hate to suggest the obvious but when was the last time you filled the truck up and did you give it some additive? Up until last week, we have had some pretty cold weather. Perhaps a partial wax build up in the filter??? Around here, winter diesel didn't hit the pumps until mid November.
Have you done a power test? If it pulls well up to a certain point then falls flat on its face, typically this is low pressure fuel delivery (filter, lift pump etc.). A gradual loss of power over the whole power band, hard smokey starts etc. typically points to the IP.
The pumps are not really hard to overhaul, even if you have to replace the head, however, you do need a flow bench to set them up properly. Probably easier and cheaper to get a good reman. It also good advice to replace the squirts at the same time and setup pop pressures to match.
Probably not what you wanted to hear but I would be absolutely anal about checking the fuel delivery upstream of the IP before condemning the big ticket items. Spent a butt load of cash on my first 5.7 Olds, only to find that a new filter I had replaced had gotten a load of wax into it and had collapsed. Couldn't be the filter....I just replaced that
Just as an aside, my 89 snowplow gets the same symptoms if I forget to put in the additive. The old DB2s do not like the new ULSD. Add some kerosene for the winter and I can hardly get it home from the gas station. Like pumping sand through the IP.
Bill