Whats the problem with my car? please help!?

Hi I have a 1999 pontiac firebird...yesterday when I was on the freeway the car started shaking (kinda like how it feels when your on a flat)...my tires where fine though it felt like the movement want coming from the engine, I am not sure tho a few weeks back a mechanic told me I need a cadirac converter...can this be the problem or somthing more serious...please help thanks...also, the car is still running but it still shakes...thanx again..