1 piece drive shafts came on the 4wd models, being the transfer case takes up space. My 73 is a 2 piece also, but I plan on getting a 1 piece aluminum in the near future. Is your father-in-law ' s drive shaft steel or aluminum, if it's a aluminum I would say it's been swapped at some point or custom made, if steel, is the truck 4wd? If not 4wd, probably custom made too? Not sure on exactly when GM started using the one piece aluminums, 2 piece steels are still used today on newer trucks. Hope this helps.