A few weeks ago while I was replacing my exhaust, I noticed a band of grunge on the gas tank and areas surrounding the pinion seal on the rear end. Increased rear end noise was not noticeable, and there was never a spot of oil from it in the driveway that I noticed. Since the rear end uses heavy oil, leakage probably only occurs when it is rotating, so would be hard to notice otherwise. I don't know if pinion seals normally leak on 11 yr old/130K cars, but probably worthwhile crawling underneath the car occasionally to check for among other things.

Tom