No reverse

I recently bought the truck listed above 2500. It wont go into reverse. All other gears work fine. When shifted into reverse the truck will power down like it normally does, it just doesn't move. The guy told me that one morning he couldn't get it to go into reverse so he just yanked on it til it did. Never worked since. I don't want to buy a whole new transmission because I don't think it's necessary. The transmission is a 4l60e.