Look at all the attachments in these threads; you need to be 100% certain of the ratio of the diff you have plus the diff you want to instal:

Subaru Diff Clarification - Drivetrain - HybridZ
Difference betwen R160, R180, R200 rear diffs - NASIOC

 

I still can't fully understand. It seems like there are multiple conversations happening in those threads. I believe that I have a 3.545 r160 diff in my stock 02 wrx.. Please correct me if I am wrong. I would jut like to find a bolt on donor. Does an Impreza RS have the same diff? From what I understand, a ratio can be the same, but the actual size or bolt/spline patterns can vary from other cars. This will cause me to have to buy new axles and possibly a a driveline to match if it isn't the same-just to make it work..

 

I've seen charts that compare all the data you would need, but I'm really having trouble finding them. I've been down a similar road and it's really not worth it from a common sense, effort, or financial point of view.

If you want a direct replacement, go to the interwebz, search your year and model, buy from a parts yard. That's the cheapest and easiest way. Or go to local dealers/shops and ask around, usually those guys have some better connections and could get you something for a good price.