I had the axles replaced at 130,000 miles and sometime after that I noticed
a slight shimmy/shudder while under acceleration at about 23 miles an hour.
Any ideas? The vehicle has had reasonable maintenance and is in great
shape otherwise. Motor mounts, a bad replacement axle?

 

Motor mounts and axles are both candidates.

The motor mount will tend to transmit an engine frequency vibration. At
23MPH, that should be significantly higher freqency than the wheel speed.

A bad axle joint or an axle that's too long will tend to create a wheel
speed vibration, often only at certain speeds and/or only on
acceleration/deceleration such as you describe.

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