A friend of mine had a 4speed that was having problems in reverse and it finally wouldn’t work backing up. We never toke it apart but you could tell that it and lost all the teeth on the gear as it would grind until it hit good teeth then slam it would backup 4-5 feet and then grind again. It got worse and worse until he didn’t have reverse at all. We switched in a 5 speed that I had and every thing is good now. good luck