Funny feeling i removed the big metal plug to change the seals when i did this but i could be wrong, it may depend on what i received in the kit as i seem to remember there are 2 seals on that input shaft, but do you also get an o ring and a bearing in the kit as well?

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Personally i would use grease or ATF rather than wd40 for install and make sure its the correct seal too as that box has been around for a long, long time on various vehicles so unless you are certain it came with the CJ and hasn't been altered then it could simply just be the wrong seal kit?.
If you are feeling brave then its not that big or horrible a job to remove that plug especially if the box is off, makes access easier for emery cloth too.
So long as you make sure to scribe/paint a line as to where it was all tightened up to before you undo the locknut and plug and that way it all goes back to where it started from when reassembling.
Oh and dont get tempted to turn the input shaft with that plug off as it can unscrew out and thats when all the ball bearings fall out and in to the steering gear and thats not a good thing believe me!.