Hi Tim:
I see this post is quite old, but I am curious if and how you solved your problem. My '38 half ton has the same gear box and I have it at the machine shop now with the same puzzle you had. I found a NOS pitman arm kit that had two bushings and the split bushing. Near as I can tell from a note in the Master Parts Catalog, there was a problem with chattering and the new kit solves the problem in this particular 37-40 box.
Cheers,
Mark

One who knows that enough is enough will always have enough. Lao Tzu