With mine it was pretty obvious. The bearings fell out of the race when I took the king pin out. Because of the way the knuckles are positioned, it's easy for dirt to get caught and wear on the seal. A new seal may be all that you need. But you won't know until you take it apart.
FYI-some of these bearings can be preloaded. You might just need to snug them up a bit to get rid of any slop. The roll pin was missing from the castle nut that tightened my wheel bearing. With the extra slop it didn't take long for the seal to fail. Depending on how long it would have gone, the damage could have been destroyed gears, galled bearings from the lack of lube to the nut falling off and me actually loosing the wheel! Although these axle look real complex, they're actually not that bad to take apart. Just do one side at a time and lay out the parts in order so reassembly will be easier. I was able to get all the bearings and seals directly from a bearing supplier. I replaced every seal, a couple of speedy sleeves for the wheel seals and a couple bearings for $100.
