2002 Jeep Liberty Speedometer

My speedometer is acting erratically (ie: not stopping on a speed, shooting down to zero). Then it finally went bonkers and the car died. My mechanic replace 2 crank sensors (whatever that is)! The problem is still occuring although the car is not dying out. What could this be and is it dangerous to drive? In case you havent figured it out, I am a woman who knows nothing about cars! Thanks for any information.