Hmmmmmm.....
I've seen K5s come stock with 3.08s, 3.42s and 3.73s. In fact, my 84 came from the factory with 3.73s. If yours came with a factory-installed receiver hitch, chances are you have the 3.73s, too.
If you still have a readable RPO sticker, it shoud tell you what gears you have. Gearing codes start with a G, and if you google "RPO codes", you'll find a ton of sites that can help you decipher your sticker.