I have a 2001 legacy L automatic with low miles (20K). Car has significant vibration that is especially noticeable between 45 and 50 mph under ALL conditions - accelerating or coasting. Vibration gets less noticable as speed gets higher but never really goes away. Kinda feels a little like you're driving on grooved pavement even if surface is smooth.. or kinda like driving a manual car when it is in the wrong gear ( 1 gear AHEAD of where you should be)

Dealer tried swapping axles with another car, swappping wheels with another car and swapping driveshaft with another car.... still no solution. Planning to put in new transmission.

Is this a waste of time? Only other thing I can say about car is that idle does seem a bit rough sometimes. Slight hestitation when accelerating from a stop. Could there be some other issue lurking under the hood thats causing these symptoms like fuel system issues or idle/air-fuel-mix isssues?

Thanks !
rob