All the above are good suggestions. As for the mounting plate cracking, it does happen fairly often. You should look at the mounting plate from the bottom up. Look at the "curved sections" of the plate... that is the sections that have some contour. Use a good light and look to see if there is any sign of slight cracking or signs of "fatigue". Some are hard to spot in the initial stages, others are progressed enough to be obvious to the naked eye. (is there another kind??)
You may find that the only way to see a failure is to lift the front end, and cycle the steering from "stop to stop" since this thing usually needs some serious load to betray the flaw(s)
As for the steering input shaft or 'stub shaft' it does have a seal that can be changed without removal of the steering gear. (Many thanks to Case500D for the extra tips/details when I did mine)
DC has a repair kit.
I have concluded that the most common reason for this to fail is the steering coupler, which usually fails due to lack of lube, and contamination getting in from the rubber boot.
If you get the seal kit, and a new coupler you'll be good for a while, but it will likely recur.
Many of us here have gone to the Borgeson steering shaft with good results. I don't know if anyone makes a comparible item but it can't hurt to check it out.
bob.