Okay, first things first, that is indeed the fill plug. Oil around it does not necessarily mean that its leaking, as dirt and crap on the back end of the diff will act like a wicking material and soak up any oil that's around the plug itself, this is pretty normal. A good spray-down with brake clean and a quick wipe of the back end will get rid of the oil.
Usually you'll see this happen when someone does a diff service/check, which should be part of the maintenance at, IIRC, every 30k, at the dealership. Basically, they pull the plug, see if a slight drip comes out of the fill hole, and make sure the oil is not milky (which means someone's been fording rivers/streams without a diff breather relocation tube!).
Because the diffs are usually overlooked by most owners, its should ALWAYS get a new crush washer in there whenever the plug is pulled. This will keep it from leaking.
Now, you will still have some oil seepage, mostly from the plug since the hole is dripping oil when you put it back in there. That little bit of oil on the plug and crush washer is plenty enough to wick all the way around the plug, exactly like your photo shows. It may take a few weeks to months, but it will do this nonetheless. Nothing at all to worry about.
One other thing here, and this is why the dealership says its a "common thing with the offroading crowd", is the fact that when offroad, vehicles will see different angles, such as going uphill and downhill (which would affect the front diff), that puts the oil level directly over the fill plug. When combined with heat from gear friction, as well as some oil getting into the breather, a slight amount of pressure will build up in the diff which could potentially push a bit of oil through the threads of the fill plug, and will bypass the crush washer if its not been replaced each time you check it.
These are all such slight amounts of seepage that it would literally take decades to really affect the oil level in the diff!
Oh yeah, there is no reason whatsoever to have sealant put on the fill plug for this issue! IMHO, this is just a bandaid on a problem that doesn't exist, and the service dept is just appeasing you to get you out of there.
Next time you're in a parking lot, take a look at all the rear wheel drive vehicle's diffs, I'm willing to bet that most of them over 3 or 4 years old will have the oil spot on their diff.
I just changed my diff fluid in my '87 truck about a month ago, cleaned it all up with brake clean when I was done, wiped it all down, used a brand new crush washer, and with the dust around here (I haven't driven the truck in wet conditions for years so its always dusty underneath), I have a small oil stain just like yours, both front and rear.
