ha ha! I am thinking about the exact same thing. From what I have gather so far. it should be okay... but here is the deal.
02RS MT has 4.111 final drive
02WRX AT has 4.111 final drive
02WRX MT has 3.9?? final drive
so that means you have to find a AT rear diff off of a 02WRX. BUT if you just swap the LSD with the Open Diff, that should work too. But either way I think you need the rear axles too.
I think most likely that's what I am going to do. But I need to findout if the ring gear off the 02RS will fit the 02WRX LSD or 00-01RS LSD. After that, gotta make sure the axles will fit also. I am not sure about the 00-01RS LSD and axle on 02RS. I think I am going to find a 02WRX LSD and alxes. Take the diff apart, take out the LSD and swap it with my open diff and use my ring gear and pinion gear. If all that fits...
the 00-01RS has LSD and someone on i-club said I could swap from them... I am not sure how that would work. I need to check the part numbers and the shop manuals next time when I am at the dealership... which would be a 3 weeks from now....