I have never figured out how to deal with a worn 47-55st 3-speed "shift box".
However, my ignorant suggestion is to carefully take apart the box: being careful to keep ALL parts/shims/etc in order.
Carefully, clean everything (keeping everything in order and in correct orientation).
Put the assembly together, very carefully, coating everything with very little chassis grease.
Fill the "shift box" with chassis grease (I think that is the correct lubricant - some will confirm/correct this).
Hold your breath, say a prayer, and maybe it will smoothly shift (and not get "locked-up" when shifting).
You lose nothing by trying this suggestion.
A few decades ago, I bought a rebuilt unit from Jim Carter Truck Parts - it looked terrible (amateurish) and worked terribly.
I changed over to an SM420 4-speed floor-shifter. I still would rather have the three speed.
Please let us now how this proceeds.
