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No I don't believe that it is but since the shaft is tapered all you need to do is to tighten the bolt. I believe that the torque spec is over 100 lbs for that nut, BTW.
Do you have a lot of play in the steering? Anyway, if the arm just seems looose, try to tighten it first. Stage two would be to replace the box and since your truck is a '78, it might be aboiut time.
I do have very sloppy steering and found the loose arm while checking everything the book said to check. the steering box does seem to be weeping out of the top (the opposite side of where the the arm bolts on). Is the steering box available from a parts place and anyone have a ball park price?
about 150 with a core charge of about the same. Not too bad of a job though you need a pitman arm puller. Helps to have a second set of hands but I did it by myself in about 1.5 hours and in no rush. I got mine from autozone and am extremely happy with the results.