Speed sensor van is jerking and not reading the speed

There are 2 sensors on trans if it is a 604 (4 speed overdrive trans) 1 is an input speed sensor the other is and out put speed sensor. You can tell which sensor it is by hooking it to a diagnostic computer, either dealer or most after market computers such as snap on or otc. Read codes it will tell you which it is need to be in transmission code not engine codes. If no computer is availible get an input speed sensor and replace it. Most of the time it is an input sensor that fails. Located in front of trans it will be the sensor on the left side. Takes a 1 inch socket to remove it. They have differant threads so you wont get it in the wrong hole.
Good luck