Vibration...

The most common cause of this is an out of balance wheel or wheels. I would check for missing wheel weights and even if you don't notice anything missing, have all the wheels rebalanced. Just a quick question; have you recently rotated the tires front to back and back to front after not having rotated them for an extended period of time? I would also have someone look at all the suspension components. A worn suspension component can cause vibration at speed.

Mike

I was going to say tires also. I had some uneven tire wear on my other car (due to worn suspention parts) and it would shake like crazy.

thank you very much for advice. One more think that i noticed is that i feel strange sound while i was on high speed while i dont feel any problems at low speed. If the vibration result from the wheels, should this problem make strange sound? Would the engine itself make such sound when we are on high speed? Thanks

Hint: when you have the tire-wheel combo balanced, have the tech watch for lateral run-out, wobble, tire defects, wheel defect and tire eccentricness <<new word<.

Even a perfect balance can not mask a bad tire or bent wheel.

Do a couple of searches for balance with 73sport as the user. . .
FWIW my cami had some totally shot inner tie-rod ends and did not vibrate at all because the tires had a good balance!

Shocks and suspension help control motion and direct motion. They can not fix bent wheel or an out of round/out of balance tire(s), although they will help MASK IT until the vibrations wear them to point of making the problem(s) obvious.