I have an 04 WRX wagon that has a pretty bad shake at highway speeds but only under load which seems to be coming from the back left. It's felt throughout the car, the seats shake and the shifter aswell, not so much in the steering wheel. As soon as I let off the gas it smooths out. I recently replaced the tires/balanced but the vibration is still there. I inspected the axles, they seem to be good too. It's not consistant and some days it happens and some days it will be smooth. I can "feather" the gas once it starts to shake to get teh car to smooth out so I'm not sure what could be causing it. I haven't had an alignment done yet but I'm not sure if that would cause it to be intermittant like it is. Any ideas on what to inspect? It's very annoying and makes me not want to drive my car on the highway.