That is the carrier bearing for the rear driveshaft that has come apart - - it is there to reduce shaking and vibration. The seal probably came off from bearing failure and it finally gave out. Ive never done it, but I have heard that they are not easy to remove and replace without a press and the right dies. It might be best to pull the driveshaft out and bring it to a driveline shop to change it. The carrier bearing itself is probably only $20, but its worth it to pay someone else to do it to avoid the frustration of trying to do it yourself without the right tools.