I wouldn’t muddy the waters by getting into any further detail beyond “it’s a 1984 K5 Blazer, J code diesel”. That’s all a competent parts counter person needs to hear to help you. I’ve sent my wife to get parts on more than one occasion and that works for her.
Starting down the path of explaining what a CUCV is and how yours may not have been manufactured in 1984 will just confuse most people. Of course, if the parts person is interested in knowing I’d offer to explain, but in my experience it just confuses the non-enthusiast and makes the transaction difficult.