After reading what was involved with replacing this seal, I was thinking, as you suggested, to let her go as is. Thinking, what would be the point of spending time and money on a seal when I'll probably be replacing the whole diff. anyways.
What signs would I look for that would indicate that the diff. is getting worse? Would it be a louder whinning sound or will something let go and I'll start hearing grinding sounds. Basicly I am asking, is this something that would lock up on me and leave me stranded. Also thank you for the advice, you have been a great source of info. for other problems I have had.

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1999 MB SL500 (110,000 mi)
2004 Volvo V70 2.5T (220,000 mi)
2014 Tesla Model S 85 (136,000 mi)
MBCA member