I don't know of any widespread trade of salt in pre Columbian Midwest. However, salt is readily available across most of it from my understanding. There are several salt mines around the great lakes currently, the biggest I believe being under lake Erie. If I'm not mistaken there was also a lot of salt brought with the glaciers, which is why so many rivers in Illinois have consistent salt licks along their banks. I don't know nearly as much about the plains regions west of the Mississippi, but would guess that it's similar.