As far as I know, '79 was the first year of the Toyota 4x4 in the U.S., you could have a converted 2x, or a '77 body/interior on a '79-'83 frame. If it is a factory 4x4 it should fall under the '79 model year, the production date could be 09/77, but its model date could be the '79. My '90 has a production date of 12/89, and my '91 has a production date of 1/90, production date doesn't alwasy correspond to model date. If it is registered as a '78, some dealers may have gotten demo models prior to them going on sale as the '79 model year release.

If it is a factory 4x4, you should be able to use '79 parts, if it is a conversion/cab swap you should have better luck requesting parts if you say you have a '78 2wd rather than a '78 4x, and only mention that it is a '79 4x when requesting parts for the drivetrain.