Pinion Seal. $10 part, but can be a B***h to replace. Lots of tutorials online and they looked super easy. I recently did my front one and had to use a floor jack and breaker bar to get the pinion nut off. Then almost as impossible to get it back on and lined up to where it was before. If you can get the pinion nut off with just a breaker bar, then it will be a breeze and I would say go for it, otherwise have shop do it.