So here are the symptoms again hoping someone who had a similar problem can help me out. (This is not an alternative to taking it to the mechanic, but since I don't have time to go to the mechanic now then why not ask online?)
No shake when idling. And no shake between say 30km/h and 80km/h. (These numbers are not set in stone--they are just approximations.)
There is a shake at low speeds; 30km/h and below. It's almost like the car wobbles, as if one tire is bigger than the other tires or something! But for some reason I can't help but think it is coming from the centre of the car!
There is a slight (negligible) shake at 80km/h and above, but it becomes notable at 120km/h and above. I feel the shake mostly in the floor of the car especially in the gas pedal. But it's also felt slightly in the steering wheel; the steering wheel shake could very much be due to the worn out thrust arms.
I've almost ruled out tires, wheels, rotors, and brakes as the cause because each have been inspected many times. I am hoping for a solution within 2 weeks because I am planning a 4hrs long trip and I don't want a shaking car at highway speed! Please!