They aren't too bad to replace. Once you get the axle out the seal you want to replace is in the tube. A couple of things come to mind, since gear oil has been leaking through the axle bearing you should repack it with grease. I use a "needle" fitting that works with a grease gun. Second, more often then not, the tension spring on the inside of the seal lip jumps off while installing. You can glue the spring one by coating it with grease before driving it in.
