I dunno....when you have a vibration that you can "drive through", tires are a prime suspect. Do you feel this in the steering wheel, or only in the "seat of your pants"?
I suppose you could check all components being driven by the serpentine belt, for being bolted down properly.
If the vibration were from 0 mph on up, I might suspect drive axles.
Usually wheel end vibrations (tires, wheels, etc.) are in the 50 to 70 mph range. Since this is 25 to 40 mph, I'd suspect driveline - u joints in particular.
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I suppose you could check all components being driven by the serpentine belt, for being bolted down properly.
If the vibration were from 0 mph on up, I might suspect drive axles.