Have you determined why it is leaking? Mine developed a leak last year and I was able to order the parts and repair it myself without all that much trouble and no special tools other than what a regular mechanic might have. The only "special" tool I needed was an impact driver for loosening a couple of screws. Seriously, take a look in your manual at how the final drive is put together and you'll see that there are only a few seals that can allow leaks. I replaced them all for not that much money. I don't remember exactly how much I spent on parts but it was less than $50 and I did the work one afternooon in my driveway. On a scale of difficulty 1 to 10, I'd rate it not too bad.
P.S. Not to point too fine a point to it but Goldwings don't have a "differential". They have a "Right Angle Gear Drive" for their final drive. (If I've mis-called it I'm sure someone will correct me. Please.)
Honda of Van Wert
10852 US 127 South
Van Wert, OH 45891
Service Dept. Gary Perl ... Phone 1-419-238-1776
These people are a trike conversion shop as well as a GREAT Honda dealer ... they recently replaced my 1800 rear end when it suffered a "catastrophic" failure when the main bearing went out cousing a BIG leak in the seal and almost ALL the oil spilled out instantly.
My left ball bearing failed also. The cage was destroyed and it cut the seal completely out. Dealer took the damaged final drive and gave me one from a triked 06. All is up and running. The bike was down about 27 hours.