This is not common, and more than likely due to something other than parts failure. As evoking stated get a new case, and build it per the instructions. As a note DO NOT run a Locked Diff in your ET48, it will just cause something else to fail. the center diff is meant to distribute power front to rear, and a locked diff no longer distributes as needed, just straight drive. You will see higher wear, should you run it locked.
I have run the ET48 since the original up until the ET48.3 and have never come across any abnormal diff wear. If you want to be extra safe, and prevent it you can run the Aluminum Diff Cups as a precaution, and this will be avoided in the future.