Result: amplitude of wobble around 55 MPH vastly reduced.
It seems that having your tire pressure too high can contribute to the wobble, even cause it in some cases. I've habitually kept the tires at 34 PSI cold, which goes up to about 38 PSI hot--and that's too much pressure, I think. So from now on I'll try to keep 'em around 28 PSI cold.
It's not gone but it's lessened, and I'll take any improvement I can get.
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Seen on YouTube: