[Tireiron]

The front diff of all 6MT transmissions is a 3.9 ratio. The older ones use a 1:1 center ratio requiring a 3.9 rear diff. The newer ones use a 1.1:1 center ratio requiring a 3.54 rear diff.

The USDM (I'm not sure if the overseas WRX got the transmission change) 06-07 NON STi WRX 5mt uses a 3.7 front diff with a 1:1 center ratio requiring a 3.7 rear diff.

Also, the SVX axleshafts in the rear are unique, however that being said you could technically swap from a 3.7 SVX rear diff to a 3.54 SVX rear diff without requiring any diff work. However the better option is to swap to a STi rear r180 and the STi rear axleshafts. The r180 is available in either 3.54 or 3.9 depending on the transmission you use. And with the r180 the 05-07 USDM 5x114 STi rear axleshafts will bolt in.

*I specify USDM because the years change between JDM and USDM markets because if the USDM auto maker release schedule.