Does 4x4 reduce payload on a diesel truck?

Yes, 4x4 can reduce payload because it adds weight. The front axle parts, transfer case, driveshaft, and related hardware all count against the truck's available payload. The difference may not matter for light use. It can matter a lot if you are shopping near the limit for a fifth wheel, slide-in camper, or heavy gooseneck. That is why you need the door-jamb sticker on the exact truck, not a generic rating from a brochure. 4x4 also adds parts to inspect. Check the transfer case, front differential, axle seals, hubs, driveshafts, U-joints, and whether four-wheel drive engages correctly. Look for leaks and front-end wear. Still, 4x4 is worth it for snow, mud, job sites, boat ramps, rural roads, and resale in many markets. Just do not assume it is free. It costs weight, money, and maintenance.