1998 CK 1500 4X4 Speed sensor buffer location

[Mvovr1]

Every article I read says the speed sensor buffer is behind the glove box above the computer. Yet I cant find it? My speedometer is reading 20 MPH faster than I am going. I just purchased the truck and I want to set it back to factory spec. Has anyone experienced the speedo being so far off? If so I'm up for ideas how you made your repairs. I was planning on starting with replacement of the sensor. I have a 5,7 with automatic 4L60E Transmission. (I think)

[Mvovr1]

I am going to replace the VSS on the transmission first. Has any one ever experienced the VSS causing the speedometer to be 20 MPH off with a factory tire size?