I know you already replaced the diff bushings, but, I've read that there are 'helper' pieces and maybe full aftermarket poly units that go in there. maybe Whiteline or prothane, etc.

 

Also, I believe cardoc over on the outback forum used 3M windo-weld to create some bushings.

 

just mentioning in case someone else reads the thread with diff bushing problems.

bad pinion bearing or bad side bearings on the diff carrier will also cause a nasty vibration. I had a customers legacy in the shop yesterday that had a bad vibration/whine around 45-60 mph, everything checked out ok, except the diff was low on oil and stinky. flushed/replaced the gear oil and it quieted down some and most of the vibration went away.

Too bad the pics and vid don't work anymore... my 1998 outback has similar bushing problems.