my girlfriend bought a( Fix Or Repair Daily )OR FORD i am a Chevy man but i looked at her truck's differential. Her mechanic said that her pinion gear was going bad now i talked to mine and he said that they don't just go bad they either go out or don't. So i took it upon myself to look at it i took the back plate off but everything looks good in there no metal shavings gears look like new so what could it be it also looked like there was motor oil in there not gear oil don't know if that matters. I would not mind some suggestions on what to check.