In East Alabama, where I came from in the middle of the last century, fixing to was not entirely synonymous with going to: it signified about to, on the verge of and would not be used of long-term plans or expectations. Thus:
She's going to be a pediatrician when she grows up, AND
She's fixing to be a pediatrician after she gets her degree,BUT NOT
✲She's fixing to be a doctor when she grows up.
And in fact the story you reference says that “the church was already reserved [...] and the minister all lined up, and the invitations sent.” Likewise, a little further on, the two-by-fours with which Sean busts up the car and Roxanne busts up Sean come from “a big pile of treated lumber that they got on sale from Wal-Mart” because “they were fixing to build a deck with” them and “Sean's brother was going to help them build the deck”. In both instances plans were pretty well advanced.
✲ marks an utterance as unacceptable
I understand it to mean "preparing to". I've used it before, but not often. I've used it informally when talking to friends to show my excitement about something: "I'm back in school! I'm fixing to finish in two years and get my degree!" It is a colloquialism, which English abounds in because it is spoken all over the world. – Nick – 2017-10-29T18:05:38.703
@snail, as a non-native speaker, shall I consider "fixing to" a colloquialism, a new tendency in the language, an American form, or what? Am I safe in using it or would people frown at me if I did? – None – 2013-03-22T22:42:37.567
2I wouldn't use it if I were you. It's definitely not "a new tendency", but it's not accepted as part of Standard English. The most I can really say is that it belongs to dialects I don't speak. I'm not sure exactly what it means, which is why I didn't describe it; my feeling is that it means going to, but dictionaries say preparing to. – snailplane – 2013-03-22T22:46:03.527
2@snailplane I don't use it, but those around me do. In my experience it means "a relatively short time from now, I am going to do [x]". It's interesting that dictionaries define it as "preparing to" though. That's not how I've heard it used, but it makes more sense as to the development of the phrase; "fixing to" could be extrapolated to "I am fixing things up in order to be ready to do [x]". Interesting!! – WendiKidd – 2013-03-23T15:42:05.120