I have a 1993 Civic Si... and once I reach over 130 km/h the car starts shaking and jerking...
I just want to know what the major problem is... Is it the transmission? the Wheel?

 

Since that is a problem that can really only be properly diagnosedby seeing/feeling the car it's hard to know for sure.

My first guess would be that the wheels are out of alignment. Solution: get an alignment.
-to check if it may be the issue: drive on a straight road, aim the car straight, then let go of the steering wheel. If it pulls a side, it's out of alignment.
Don't forget to grab the steering wheel when you are done checking the alignment.
-Note, even if it doesn't pull, the alignment may still be off.

My second guess would be that a wheel bearing is bad. Solution: replace wheel bearing
-to check if it may be the issue: Jack the front of the car up. Use jack stands to secure. With the wheels still on the car, grab the top and bottom of the wheel and attempt to wobble the wheel (check for play). If you can wobble it (if there is play) then the bearing is shot.

My overall suggestion for you is to take it to a certified mechanic that can do alignments and ask them to do an alignment and check the wheelbearing and inspect the suspension while they have the car.
Best of luck!

 

Thanks a lot.. Ye I checked it this morning... I went straight and let go the steering wheel.. and the car was moving towards the right side... So I guess its alignment.. Ill get it fixed sometime this week
Thanks