For an R180 swap, you'll need...

R180 Diff - If your going RWD, then you dont need to worry about getting the right ratio. but it would be a good idea to match up the ratio with your gearbox so that if you go back to AWD then at least you can leave the R180 setup in there.

R180 drive shafts

and R180 hubs - you also will more than likely need go to brembos or 2 pots as the larger R180 hand brake assembly cant be used with the WRX 260mm single pot caliper.

Best thing to do is find a complete gc8 V3-V6 STI-RA R180 rear end assembly all attached to the sub-frame with all the suspension linkages, diff, hub, shafts and brakes and just bolt that onto the car. Makes life alot easier. Expect to spend around $2-3K on a setup like that. This will mean you will be stuck with a 4.44 R180 diff, which unless you have a STI-GC8 gearbox or a 02-07 WRX gearbox, wont work in AWD. if you have a normal WRX gc8 gearbox, and you wish to have the capacity to convert back to AWD you will require a 3.54 R180 diff, which you can find in any 06-07 STI.

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RS Liberty - Because WRX's are only good for parts...