What do you mean it “always” has prefered to run on fewer than 8-cylinders? It’s done this since it was new? I’m assuming that there was some time in the truck’s life that it ran fine. What makes you think that it’s only running on a few cylinders as opposed to all of them poorly?
Cars nowadays can’t just drop cylinders here and there like old ones could-- there would be catastrophic consequences to driving it around for any length of time with a whole cylinder consistently missing. Is the “cylinder” perhaps just your unit of measure for how well the truck is running?