AT Temp Light, Won't Go in Gear.

2006 RCLB 4x4 4.7
Today I had been driving the interstate 80 MPH, no trailer and I pulled in to a gas station, went inside, then got back in the truck, started it up and the "AT Temp" light was on. I put it in gear and it wouldn't move. Engine running fine, just wouldn't move forward or in reverse. I put it back in park, shut the engine off, restarted it and everything was fine, light no longer on. I drove another 130 miles with no problems. After I got home and let it cool off, I checked engine oil for coolant and radiator for motor oil, but everything was normal.
It has 265K miles on it and the transmission is well maintained. Last month I added 4 more quarts of WS ATF , idled 10 minutes, drained it, added another 5 quarts and let the jack down and pulled the plug on the standpipe with the engine running in park and also replaced the Magnefine filter because it had been about 40K miles since I did that before. This transmission has never slipped or let me down before.
Anyone else had their AT temp light come on? Any ideas?

 

What procedure did you use to determine whether you had the correct fluid level during your drain and fills?

 

I drained all that was in the pan, put the plug back in, added 4 quarts, ran it at idle 10 minutes, shut off and drained all that was in the pan, put the plug back in, added 5 quarts, ran it 5 minutes, let the truck off the jacks level on the garage slab and removed the standpipe plug with it idling in park and about a quart drained out, then put the standpipe plug back in. I have driven it over a thousand miles since with no problems except that incident yesterday.