TRX 300FW Rear Brake Issue

21 years after getting this bike , I am finally replacing the brakes. Front went fine, but one cylinder is stuck, but I can live with that

My issue is with the rear brakes. When I pulled the brake drum off, it looks as if oil has seeped into the drum (no wonder they didn’t work). I assume there is a seal gone bad between the differential & brake drum? Any pointer as to where to start looking ?

Thx

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