4L80e transmission not stopped shifting properly while driving

I was driving the vehicle on the highway at about 65 MPH, when suddenly the transmission shifted from 4th to 3rd gear and would not go back into overdrive. After having stopped but not shutting off the engine, I continued on my way home, which was only 7 miles away, the transmission shifted only to 2nd gear and it is now stuck in 2nd gear only. I disconnected the negative battery from the battery and turned on the light switch. Left the light switch in the on position for about 15 minutes, in attempt to reset the computer. Went back, and turned off the lights switch before reconnecting the negative battery cable and restarting the vehicle. After having done so I took the vehicle for a drive and still no change in the transmission shifting into out of 2nd gear. I had a transmission repair men put a scope on it to check for faults in the transmission and he said that it was not showing any fault with the electronics in the transmission. Could it be that the ECM is faulty?