Leaking pinion seals are very common on these trucks. I noticed mine was leaking recently. I did a bunch of searching on this site and came to the conclusion that replacing the pinion seal was a job better left to the pros. You've got to get the pre-load just right. Too tight and you'll ruin the bearings. Too loose and the pinion gear will move forward and back. A good mechanic should be able to knock this out in 45 minutes or less. It shouldn't run more than $150.
If you decide to do it yourself, you'll need a new pinion seal and a new crush sleeve. I think Ford also recommends using a new pinion nut. You'll also need an inch-pound beam torque wrench to set the pre-load. BTW, if you set the pre-load too tight, you have to start over again with a new crush sleeve.
The diff should be warm if you've been driving the truck. There is a bearing in there (a couple of them, actually). Once you have the pinion seal out, I think you can get to the front bearing and inspect it. Good luck with it.