rear o2 sensors only monitor cat efficiency and have nothing to do with the engine running rich or lean. you have a rich condition which means you either have too much fuel or not enough air. I wouldn't throw parts at it yet, the first thing I would do is check for vacuum leaks (pcv hoses, intake gaskets etc.).
2002 GT: 4.10s, AM CAI, UPR o/r X-Pipe, MGW STS, Flowmaster 40's, more to come