I don't think anything bad happened to the driver, fortunately. From the few things the various people I talked to said, I think he was a temporary driver (they referred to a "contractor" several times) and just decided he didn't like the job so he left the truck somewhere and went on his holiday vacation or something. If he'd been carjacked or something then I'm sure the contents of the truck would have been stolen - something I very grateful did NOT happen.
From comments the guy who did finally deliver made to me, it sounds like the original driver was pretty lazy and just couldn't be bothered to try and deliver any packages that took an effort. The final driver actually came down to Phoenix from one of the small northern towns just to do these deliveries - I made sure he knew how much I appreciated his help