Something I have seen at my shop is there is a gear inside the transmission that can get damaged and cause the speed sensor to not read very well... The transmission can become contaminated with antifreeze from a failed transmission cooler.... Check to see if you have trans fluid in your coolant reservoir, if you do you may have this problem... Also I found this guy put a video on it.
https://www.youtube.com/watch?v=ej_pORD7EIY