2004 Impreza Outback Sport exhaust

I've got a 2004 stock Impreza, fixed it up quite a lot (it has 200k). Spark plugs,
belts, dash lights, filters, all the fluids, new radiator, etc. Only thing left to fix is
this little weird problem that arises on the freeway...
So when I am above 45 mph id say, if I accelerate to go up a small incline or
simply to gain speed in the car, I feel the car vibrate, especially in the gas
pedal and some in the seat. Its not insane, like its not shaking apart by any
means, its just this vibration. When I take my foot off the accelerator and coast
or maintain the speed, it stops vibrating. Tires are balanced.  I was thinking
maybe struts? I know the rear struts need to be replaced. Let me know what
you think.