So, it's all fixed. Coupla tips--
1) The 3/16" allen head screw (pn 22 at link above) in the bottom of the steering shaft was just in finger tight. That may have actually been the whole problem, but since I had already ordered and received the new shaft support, and since you have to take off the shaft support to get to that allen screw, it was a moot point. If anyone has this problem in the future, and is on a tight schedule, it might make sense to check that, tighten if needed and re-assemble.
2) I cleaned up that allen head screw and dosed it real good with thread lock.
3) I figured the easiest way to get it out was to take off the lock nut (pn 12) from drag link (pn 10) to free up the shaft assembly(pn 23), then....
4) Pop off the e-clip (pn 8) on the right side of the shaft assembly (pn 23), then....
5) Take out the four 9/16 bolts (pn 29) holding the whole mechanism on the frame. Getting those out was the hardest part. The front two are easy: socket and extension. The back left came out with a 9/16 box end slipped flat under the steering shaft. I used a conventional open end wrench to break it loose, the a ratcheting box end to get it the rest of the way out. (A magnetic parts retriever wand worked well for "sneaking" them back in.) The back right was hardest to get at, but with the e-clip off, and three of the 9/16 bolts removed, I was able to separate the assembly enough to get a long socket and extension down in there. See photo:
Once all that was done, (and the steering wheel and cowl were off) I was able to raise the whole assembly and steering shaft and get to the second e-clip off to disassemble the unit and put in the new shaft support.
PAY ClOSE attention to the orientation on the shaft support. The flat vertical slats go on the right (as you sit on the tractor and face forward; see photo:
). The shaft support has a "snout" on that side that has to go through the larger hole on the frame holding it (pn 25). I got that in backwards (shoulda looked at my own picture!) and had to take that whole thing apart and re-do it.
Also pay close attention to the orientation between the shaft assembly (pn 23) and the steering sector gear (pn 27). There are no registration marks that I could see, and I had to fiddle with it a bit to make sure it was "landing" in a way that let me get full travel on the gear turning all the way left and all the way right.
Lastly.... they shoulda made that part out of metal! Anything but plastic.
Oh.. and if I had it to do over again, I would also have ordered a new nyliner (pn 24). Mine did not look too bad, but it's less than $3, so while you have it out.....
The good news is, it all works, and if the snow hits tomorrow as predicted, I think I'm ready.