The leak is actually at your pinion seal, depending on severity it's not a huge worry provided you keep an eye on the oil level till you get around to fixing it. cspeir sounds like was fortunate with his, and the lip seal around the pinion went back to work after changing oil (and gasket). It's not uncommon for these to dribble a bit when the rear u joint begins going south.
Is there any evidence of "slinging" oil from around the back of the flange? If not I'd say just check the oil level (and the u joint while you're there) and go for now.
2004 Ranger 4x4, 5 speed manual, 4.0 V6