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Seems likely tires are at issue.
You have different sets on each axle, the tires have different characteristics and these will amplify with age, reduced tread depth, etc. possibly inducing vibration.
Tires can come out-of-round with age and this can induce vibrations.
Alignment can also go out-of-spec with both miles and tire usage, be sure this is spot-on.
Tires today seem to have a greater tendency to delaminate internally than tires made years ago. This will induce unwanted vibrations.
I suspect your answer lies with one or more of the above conditions.
Cheers!
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