I see in the Rock listings that the 91 Astro used what looks like the trans mounted VSS. I don't know what it reads inside the trans, but my first guess is that it's probably a rotor that clips onto the output shaft where the drive gear for the cable speedometer would be on your trans. Fairly straightforward, once the driveshaft is out, to remove the tail shaft housing and "reveal all." Might be a good time to add a transmission shop guy to your "support group." There's a catch term for that... oh yeah, your "network."