I know it's a completely different manufacturer but on my 4L80E when it went into limp home mode it needed a new speed sensor. The factory service manual had a troubleshooting chart and it led me right to the problem. It was specific enough that it pointed out which one needed to be replaced. Steps to repair:
1. buy replacement
2. crawl under the vehicle
3. unplug electrical connector
4. remove hold down bolt
5. pull sensor
6. replace sensor with new
7. replace hold down bolt
8. connect new sensor
9. crawl out from under the vehicle

I hope your fix is similar.

R.