You are correct that the input speed sensor is most likely this issue. However, it is inside the transmission side cover next to the valve body. It is in a slightly different location depending on which transmission you have but both are inside the cover.
I included the process below and location of both units.
Let us know if you have more questions on this. Thanks
Images (Click to enlarge)
Saturday, May 22nd, 2021 AT 12:12 PM