I am going with the information as provided by diesel mechanic. He said on the LBZ (probably not just the LBZ) GMC put a fuel tank lift pump in the fuel tank on the 3500 models but not on the 2500s. In later years IIR it was standard.
The manual fuel prime for the fuel filter had a nasty habit of losing prime. The kit with new o rings costs on $13 dollars and takes a ham handed person like myself about an hour to change out.
The lift pump cures all of that, but since I have only replaced it once, it gives you lots of notice before it fails, and have never had to do it again I haven't had the lift pump installed.