I think my rear differential is going out. I keep hearing popping noises while I slow down. I don't know if the person who owned the Disco 01 before me did a lot of off-roading in water or what, because when I changed the fluid it was nasty and milky colored. Anyways, I noticed one of the teeth was missing. I figured that is where the popping noise is coming from, am I right? How much does it cost to replace it or is this something I could do? Thanks...