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Luke, youcan buy the hub parts, individually, at most CarQuest stores. If not there, Capital Hitch, on W. Tennessee, has them.

You have to remove the hub to replace the seal. While you have the hub apart, take a good look at the bearings and races. If there is any pitting, replace them.
Re pack the bearings before you put the hub back together.

While you're buying parts, get a couple of new Cotter pins.

If you want some help, let me know. It's a simple job. Even a lawyer can do it.

Get a GOOD set of brand name Bearing Buddies (Not those cheap POS at Wal-Mart), and learn how to use them properly, and your troubles will disappear.

And like others have said, lay off the heavy greasing. You do NOT need to compress the spring all the way on the bearing buddies. Just enough pressure to move the spring a little will be enough to keep a positive pressure inside the hub. I will also get some flack about this probably, but do not buy those cheap kits at Wally World. Get good seals with double lips that are made for boat trailers and water intrusion.

Dubble

You can replace just the rear seal, but you have to pull both bearings to get to it, you may as well buy the kit, and replace bearings, races, seals and all. You will already be dirty.

You can just add grease and it will probably be fine, but thats not much better than dumping motor oil in your back yard.