Small diff leaks can make a real mess, but over the mileage between maintenance visits they don't usually loose much oil.

The easiest thing to do is have the diff checked and topped off every time you have the engine oil changed, but you will have to specifically request that from your service provider, and make sure it is written on the work order.

If they have to add oil, the quantity should be listed in the parts breakout, so you will be able to compute the actual leak rate.

I'm guessing that the diff capacity is about 1.5 quarts (should be in your owner's manual under capacities), so if it just looses a few ounces between checks, there's nothing to worry about unless you plan to keep the car forever.

Duke

P.S. Diffs have a vent at the top with a lose fit cap to keep oil from being thrown out, and these typically weep, which will coat the diff with oil over tens of thousands of mile. An overfilled diff will also tend to weep more as will sustained high speed driving. If it were me I would spray down the diff with mineral spirits to clean off the oil residue, then keep an eye on it. It could be that it is weeping out the vent then migrating down to the low spot and finally forming a drip, but you could also have a pinion seal or side seal leak.