my 2001 impala LS 3800 motor has tunred over to 96000 miles and has developed this issue when driving at speeds over 50mph's the steering wheel begins to shake back and forth and i can feel vibration from the peddle also, and can feel it a bit through the seat, and it is rougher when accelorting but it still there when just cruisin also, and i just had an alignment done and was wondering what could be causing this issue, someone suggested motor mounts, but im not sure how to do that or how much they cost to replace... anyone have any idea of what it could be?
Thanks!
~William

 

check tires for abnormalities and if good check the balance.