Thanks very much for the help, it was very interesting. I'm inclined to Dukedog's view that probably nothing much bad will come from not having that wire there, but I went ahead and drilled 2 new holes and installed a new wire, just to be sure. I have quite a bit of corrosion on that yoke already. The nuts on the bolts holding the yoke, one of which was holding the old ground wire, have some corrosion so there was no way I could remove them. Also there is a retaining ring on the bottom of the yoke and enough of that metal is corroded away that the ring had dropped down and was just lying around on the shift shaft where it goes to the footer. It was interesting drilling the holes, I have never drilled into metal that thick before, but a high speed drill bit, running at slow speed and plenty of WD40 did the job OK.
