1992 Chevy Cavalier hard engine vibration

Engine problem
1992 Chevy Cavalier 4 cyl Front Wheel Drive Automatic 93k miles

i've been going crazy trying to figure out whats causing a hard vibration in my motor, I took the car to a muffler shop to find out the catalytic converter was stopped up, . Right away, I replaced the catalytic converter, still didn't solve the problem. I also tried changing the spark plugs, still didn't solve the problem. The car is running a little better. But the motor still has a very hard vibration. The car hasn't been over heated, the oil is ok, transmission fluid looks ok. The vibration is consistent whether or not the car is in gear. The car seems to idle at a good rate, just cant figure out why the motor is vibrating so hard! My next guess is some kind of sensor, please reply soon!

thanks joe s.