I'm no expert, but I'll chime in, I just did my front end... On mine, there was a piece of the seal that broke off and stayed on the axel, like a little ring of rubber. What I'm saying is, maybe the whole seal isn't off. That's just a dust seal, so it's not a panic situation, it just keeps crap outta the axle tube.
Also, to replace it, you'll have to pound out (and ruin) the other seal mentioned above to get your axle out. Putting that funky seal in is tough without a wiz-bang Ford knuckle seal poundin' tool (or equivalent).
-dave