Well, it turns out the speedometer driven gear was OK after all. What happened: an AC Delco 90*-drive 1:1.300 ratio adapter and an aftermarket 10% reduction ratio adapter fought with each other and locked up. The speedometer driven gear, not to be denied, twisted the .104 square-drive key into a pretzel.
Oh, and the driven gear was 22-tooth, stamped "M". WTH? Had to have been replaced somewhere along the way, as the only one installed in 1978 was the 18-tooth stamped "H".
So, I replaced the two adapters with one AC Delco 90*-drive 1:1.200 adapter. It needed a .152 tang to .104 square-drive speedometer cable key on the transfer case end, and a .152 tang to .104 female sleeve on the cable end. There are some on ebay, but people are inordinately proud of them. I finally found them both with one vendor at reasonable prices.
So if you are looking for speedometer drive keys, sleeves, speedometer cable ends, ratio adapters, 90*-drive adapters etc., I can heartily recommend Texas Industrial Electric:
http://www.texasindustrialelectric.com/ No on-line ordering or prices, but they are great over the phone, the prices were less than anywhere else, looks like they have about a hundred different drive keys, sleeves, and cables ends, the parts were in stock, they shipped next day (which they were apologetic about: "Well, you missed today's mail, so they can't go out until tomorrow. Just so you know."), and they sent the correct parts, Priority Mail, for $5.05 shipping charge. I called them Thursday, parts in hand today.
Hard to beat that.