Arkie,
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.)