Door saggs on my HD3500 work truck - 1998 Chevrolet C3500

Sounds like your hinge pins are wearing out. You can get a hing pin and bushing set either at the dealer or any auto parts store. You'll need a jack and a block of wood, or some rubber matting to protect the door. Open the door and place the jack under the door and raise the jack just enough to support the door. Using a hammer, I use a brass hammer but a steal hammer will work, drive the hinge pins out one at a time. With the hinge pin driven out you'll be able to see the wear in the hinge plate, then you install the bushings required to " realign" the pin holes and then reinstall the hinge pin. Sometimes the holes are badly elongated and on the later models were the hinge is welded I remove the door and build the wear area with a wire welder. The bushing method usually works well for most. Good luck I hope this helps.