No breakdown of the box in my parts book either.
Once you disconnect the column shaft you can check the input shaft for free play. If lots of play a new seal is not likely to solve the problem If it is tight, it might be just a seal but I would expect to see a double seal in that application.
If you use your tractor a lot or need it to be ready on short notice I would order a new steering box and take the old one off for repair. If you can stand some down time go ahead and take it apart and see if it can be fixed. For me, I would take the old one apart just to see how it works. Then fix it if possible and have it for a spare. I know from another list that there have been several people with steering issues that have had to replace the box. Usually they involve poor or slow power steering.