but here's my $0.02 for what it's worth (apologize for the length)....
For the SVX, I would recommend replacing the differential fluids at every 30,000 miles or 30 months - that's following the factory severe service interval recommendation. Most people serious about the long term health of their vehicle replace fluids using the severe service schedule as a minimum. If you didn't get the books that came with the car, a search around the internet will yield almost all the data.
How much it cost varies by quality and location - where I live the cost of the fluids at a parts shop is around $10 for the front, $7 for the rear using full synthetic (the front uses more lube than the rear), less than half that for normal GL-5 lube. A shop performing this will have a markup for the fluid to retail, and of course their labor charges - that varies by locality. If you have a rear differential (AWD), you need to know if it is a limited slip model (the front is not as far as I know for all year SVX's).
What does the diff lube do? Well, the short version is that one wants to protect as much as possible against metal to metal contact because of wear considerations. The diff lube provides that protection. Additionally, it's thicker than engine oil or transmission fluid because it doesn't have the benefit of being pumped into the spots that need protection. Instead, as the car moves, some gears move through the gear lube sitting at the bottom of the case, lubricating the whole by a kind of splash effect.
All the above, except the change interval, is my opinion and you know what that's worth
