Before you do anything, clean and mark the relationship of the nut to the yoke. Use a chisel or awl and make a good and visible line across both of them. Then after you remove the nut, make a note of the relationship of the yoke so you can get it back on the same splines. Then replace your seal. Then put your yoke back on, replace the nut and tighten until the lines you made just line up and then go just a little bit tighter. Just a bit. Why? There's a crush sleeve in there. You don't have to replace it, but you do have to be careful to put it back like it was. That crush sleeve when it was installed took a LOT of torque to set.
Alternatively you can remove the crush sleeve, slide it over a piece of pipe and hit it with a hammer until it is a bit longer than it was. Then it can be re-used. It won't require anywhere as much torque to compress it as originally, and personally I'm not a fan of this procedure. But I've seen people do it and they say it worked for them.
The crush sleeve does a couple of things...pre-loads the bearings and keeps the pinion from wanting to get loose.
