Where is the speed sensor of Chevy Astro van 1991 located?

The speed sensor on a 1991 Chevy Astro van is located in the transmission. It's not easily accessible without some mechanical work. You'll likely need to either remove the transmission or access it from underneath the vehicle. A precise location within the transmission will depend on the specific transmission type (e.g., 700R4 automatic, etc.). A repair manual specific to your Astro's year and transmission type will be the best resource for pinpointing its exact location.