Is my fuel pump going bad?

[cam.man67]

So I was hauling a load of hay (about 9000lbs) behind my '87 GMC 1500 yesterday from one farm to another and the first of four hills on the way, the truck practically died. It just sounded nasty, sputtering and backfiring...almost like I had no fuel going to it. I crawl to the top of the hill, the truck comes back to life. This happens the whole way up to the farm, at which point I retired the truck for the evening and started with my dad's '08. Does this seem like symptoms of a bad fuel pump? I just replaced the left side gas tank about 3 months ago, so I don't think the fuel line is plugged or anything. The truck drives fine without any load. I've got a new fuel filter, as well as new plugs and wires, all done within the last 2 months. Any advice?
Thanks.