It's a 32mm Socket. You'll have to remove the rear calipers so if you don't have the proper Torx bits, now's the time.
The part you want is called the 'rear hub assembly'. Cost about $52 a side. I got 'em from a local Parts Plus.
Not hard to do, just follow the procedure and take your time. Make sure you break the hub nut loose with the wheel on the ground. The 'prevailing torque' hub nut requires some 'honking' to get it loose.
Also, a bit of grease on the stub shaft make installing the
new part a bit easier.
AL