first look at the axle housing (which ever side you think is leakin) if its wet around that little dust boot then yes, the inner seal is bad.. now this job aint rocket science, but it aint easy either...
-put the truck on jack stands
-remove wheels, brakes, hubs, everything all the way to the spindles on both sides...
-next, the tie rod that connects the driver and passenger ball joints, drop it outa the way, since you gotta replace the ball joints to, plus its just way easier for the next steps.
-then pull the diff. cover off, let the fluid drain.
-now pull the axles out, you will probably have to take a prybar and pry on the U-joint to pop them out.
-once the axles are out, your gonna go back to the diff.
if you look on either side of the differential carrier assembly you'll see 2 bolts per side holding on the Carrier bearing caps . just unbolt those to remove those caps that will expose the inner bearings for the diff.
-now with a prybar, pry upwards and out to pull the carrier. (keep in mind its got some weight to it, so you may need a buddy to help catch) DONT LET IT FALL!!
-the carrier may take some muscle, but it should pop right out with the proper leverage.
- once the carrier is outa the way, you'll need a long enough prybar or something that you can stick down the axle housing towards to differential housing and with a mallet tap the seal out so that it will fall out where the differential was. depending on your year there may be a plastic looking cone in the housing down towards the seal. try not to mess that thing up or you'll be replacing it to.
- once you've popped both seals out clean the area very well. brake clean works good.
-now prep your new seals by puttin a light coat of axle grease around the edge of the seals. place them in there spots and lightly tap them in so that they are snug and reset in the same place to old ones were.
-since you removed the carrier as a whole no backlash adjustment will be needed. lucky for you!
-now place the carrier back in its place bolt it down, not sure of torque specs, wouldnt hurt to look it up, but just make sure its tight without goin crazy and breakin stuff.
-clean the axles off and slide then back into place, may have to jiggle them in to get them to spline in right.
- take some real seal and put a light bead around the differential cover place back on the housing and bolt it down. clean excess real seal off and smooth around edges.
- also since your there might as well put new outer seals on with the hub assemblies and DONT FORGET THE DUST BOOTS BEFORE YOU PUT THE AXLES IN!!!
- put the rest back together as you took it apart.
- it would be best to allow the real seal to dry for a good 8 hours at least befor adding differential fliud. autozone can tell what fliud to use if you dont know..
well good luck with it all, like i said it aint a bunch of math and calculations, but it is a good 5 hours or so of work depending on your skill level. i recommend goin to autozone and tell them your doin an overhaul on your front axle and you need all the seals for your make and model.. that way you shouldnt forget anything. any more questions for more detail or whatever just ask... I'm a mechanic in the Navy so i've done this job quit a few times. still annoys me everytime too. but like i said good luck.