These Apache OEM gear boxes are what you will need if your original gear boxes are not functioning properly.
Before ordering, check the gear inside of your old gearbox. If the teeth of the gear are short and pointy, you will need to replace both of them. If the teeth are blocky (like the photo), you can replace one at a time. The gear ratio between the two types is different. This means one side will go up faster than the other if you try to use one of each.