I just wanted to close this topic out by saying that, as a few of you indicated here, the only problem was a bad cable from one of the big three. It was too long, and whether that was the sole problem, I don't know. But the new shorter one, routed over the top the engine, then down along the tunnel cover on the passenger side, out of the way of the exhaust, works fine. I have an angle drive and overdrive transmission, just fyi.
I should stop always assuming something more serious. My Speedometer is almost perfect. And, by the way, TJ's Speedometer Service here in Tucson was great. He tested it right after I walked in, and didn't try to sell me a thing. His bread and butter isn't Smith and Jaeger gauges, but gauges off more modern trucks. Interesting guy. They're not making many more like him.