1989 Chevy Cavalier wont Move

1989 Chevy Cavalier mileage: 126,000 I was at a red light, when the light turned green, I gave it gas, it moved a few feet and then jerked to a stop. It acts like it is in park; it will move a total of about three feet from as far forward as it will go to as far backward. In drive, it moves to the front "stop" point, and then acts like the brakes are locked up. In reverse, same only it moves backward. In neutral, it can be pushed through the (approximate) three feet of travel. I checked the fluid and it was low, I added about 2/3 of a quart (it was over a pint low) but no change, however I didn't check the new fluid level. I know there are several possibilities of causes, but would like to know if, in your expert opinion, what the most likely cause would be, and if it would be worth trying to fix or just replace (I recently replaced 2/3 of the engine).