1994 Lincoln Mark VIII Engine

Forget the crankshaft and sensor. A problem with the crankshaft is really serious. You'd have lots of noise and low oil pressure. That won't last long before the engine seizes up. The engine won't run with a defective crankshaft position sensor.

A vibrating engine is due to a misfire. That can be due to worn spark plugs and wires, or less commonly it is due to a fuel supply problem. Start with the basics. Replace the spark plugs and wires if they haven't been changed in a while. If the problem persists, the next step is to determine if the engine is running rough because it is running too slowly, or if it's running too slowly because of a misfire. The Engine Computer will bring engine speed back up to the desired speed when there's a misfire, but it has a limited ability to do that.

is it the same case even though when the car is higher then 2000rpm it runs and sounds the way it should?

Misfires are easy to feel at low speeds because there is such a long time between power pulses when the misfire occurs. At higher speeds there is so little time elapsed during the misfire that it is hard to notice.