Can you give us the symptoms? It may be a bushing rather than a diff problem.
Here is a good exploded view of the diff:
http://www.subaruparts.com/diag/?mod...category=195-A
The flange plate is torqued to the pinion shaft at 125-146 ft lbs so I severely doubt it has loosened up.
As for the driveshaft, it unbolts from the diff via four 12mm bolts and nuts. The driveshaft is not serviceable, but you can take it in and have new yokes welded on that accept replaceable U-joints. My last one cost $239 to be converted to replaceable U joints.
Hope this helps.
Doug