1991 chevy s10 trans issues

1991 chevy s10 trans issues hey guys I have a chevy s10 with a 4.3L automatic transmission.I recently bought the truck and the guy I bought it from said that it drives but it doesn't go into 3rd or 4th gear it has reverse and every other gear but it just doesn't shift into ant other gears.if someone can give me a couple of suggestions on what it could be I would really appreciate it thank you.