M3 differential

[PDumond21@xxxxxxx]

I have a '97 M3 coupe (5 spd) and I was told by a mechanic that my diff is 
bad and it needs to be replaced.  I have a few questions before I proceed.  
First of all, how do I know if my diff is bad?  How do I find out what the 
gear ratio is on my diff?  I believe I have a 3.23 (stock).  What are the 
advantages and disadvantages if I go to a 3.38?  How much will a 
remanufactured diff cost?  Where can I get one?  Can I get mine rebuilt? How 
much will that cost and where can I get it done?  Any insight will be greatly 
appreciated.  

Paul

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