Chris..
By any chance, do you know what the build date is for your Jimmy (i.e. just the month and year as shown on the driver's door plate)? As silly as it seems, I'm curious, just because someday, when I write my book on GM truck history, I would like to be able to note any exceptions to my generalized understanding of model year change "rules" that I uncover!
I am 99.9% sure that filler doors were not typical (key word)on '78 model year Blazers, Suburbans, or Jimmys. However, production updates are sometimes made on a "running change" basis in a model year run just prior to "formal introduction" for the next model year. I am wondering if your rig might be an example of such updating in a late production '78 model. Your picture and post are the first hard evidence that I personally have ever seen to suggest this possibility with repect to this filler door issue, so I would just like to explore this a little further!
Your picture (and Mike Reeh's and other posters' assertions) piqued my interest, and lacking access to any of my literature, pictures, and stuff at the moment, I decided to do a quick polling search on eBay for '78 Blazers, Jimmy's, and Suburbans. The pictures there confirmed my earlier impressions.
Anyway, don't mean to rankle anyone here; just curious, as I've said before!
Ken