The best thing I can tell you to do without an actual count is to grab a few different combinations of nuts. I know for sure my 88 is not fine thread, but on a 91, I am not sure. I would think that they would near the same, except for the axles being disco and non disco.
As far as knowing what the actual thread count is, I went outside and looked at a D30 I have laying around the shop...the bolt in it looks like a 3/4 inch course thread job. However I have it laying in the parts cleaner, and will probly have to run it over the wire brush a few times to be sure. That axle came out of a michigan vehicle...RUST city.
I will let you know if the bolt is salvagable here in the next hour or so...
-T