It may SEEM like its coming from the differential cover but I doubt it.
Unless you're POSITIVE it's coming from there the more likely source is the pinion seal as Ben explained.
I had this replaced under warranty also, a fairly common problem on Ranger 4x4's but not enough so to warrant a recall. Anyway I thought it was my front cover too, but after cleaning it off throughly and placing some cardboard under the truck for a few nights I determined it was in fact the pinion seal.
Either way use caution, thats an all aluminum housing, A LOT less forgiving than cast iron!!! Get a torque wrench if you do take off the front cover, you can't over tighten when replacing...Aluminum is great, when treated carefully.
2007 Liquid Gray Focus ST Gas saver
2003 FX4 Sonic Blue Sold!!! Gas guzzler best tank ever 19.5 MPG