Pinion seal is ususally the culprit of the front diff leak. Drive shaft off, buzz off the pinion nut(held on really really tight, i don't have enough air to get it off), pull the yoke, pulle the seal, repace.
Easiest way to change fluid? Get a hand pump or some sort and stick it in the fill hole, and suck all the old gunk out. Then pump new gunk in.
If the diff cover is where the leak is coming from, i'd go ahead and drop the axle out of the truck, and pull the cover, drain fluid, clean, silocone, cover back on, let dry, then fill, replace axle.
'01 XLT disguised as a '03 Fx4 Level II
Big. Loud. Slow. Bright.