I seen a flywheel that got a few rust spots and left a raised line where the disk face had relief lines in the clutch face. Its more likely a throw out bearing that was sticking on the return so the pressure plate could not apply the full clamping force. You could try doing a few standing starts in 3rd or 4th gear so it has to slip the disk on the flywheel. If that seams fine then push and release the clutch peddle fast 20 times , the truck doesn't have to be running or moving. Your just looking to let the action maybe scrape off any rust if its binding on release. I would say if you notice it slipping when your not pulling a load its really bad and should be looked at if you still have a issue after trying my abusive clutch trick.
post back if this seams to cure it , I have a guy I work with that has a Cumming 5.9L 6 speed stick in a GM truck . I could ask him if the has had any issues , his truck sits all winter also.
Mark & Melissa
2015 Redwood RL38 , Kodiak disk brakes , 17.5" tires , 8K Morryde IS