Welcome to the club Regarding rear diff a quick search on google came up with this 'There are many rear diff options, and any of them can be used as long as the final drive matches the final drive of the rear diff of the donor car the transmission came from. Front axles should be interchangeable, but newer axles are thicker. CV strength has remained the same and are not a weak point. Rear open diffs can be found on most Subaru models. Rear LSD s came on some Legacy Turbos, Some SVXs, 2000-2001 2.5RSs, and 2002+ WRXs. While final drive ratios vary, the axle splines do not. Your diff choice governs which rear axles are to be used. You will either need rear axles from an open diff Legacy or GC/GF/GM Impreza for an open diff or rear LSD axles from a 2000-2001 Impreza 2.5RS (ask for axles from 05/2000+ to be safe) for a rear LSD.' At the bottom of the post here: http://www.subaru-impreza.org/forum/modifications-tips-tricks/29706-info-subaru-engines-conversion-principles.html And a tiny bit of info here: http://www.rs25.com/forums/f4/t40400-noob-rear-diff-question.html
So seems like most are interchangeable, just check for ratios of final drive