Smoked my 4L80E at 171K; now what?

I have a 1999 4x4 Chevy 3500 with a 7.4L and a 4L80E. It has a 'work body' that includes storage side doors and a bucket lift. It has 171K miles and the tranny has died.

It was a slow death. It started with 'using fluid' and ended with smoke. About 18 months ago it came off the road to 'the ranch' where everything was 4x4 and 1st gear. On the tranny's last day we were putting up a fence. The truck was parked at each corner and used as the solid point to attach the come-along and the fence stretcher. As each corner was finished, the truck was started and driven to the next corner. At each move the tranny would hesitate to go into 1st gear from park; and by hesitate I mean you would have the rev the engine up to 2k-3k rpm to get any motion. Once 1st 'engaged', the gear seemed solid (no revving needed to keep going). After the last corner, no amount of revving the engine would get movement and after a bit of frustrated revving, smoke was produced. Adding more fluid did not solve the issue. We used the tractor to drag the truck back to the shop.

So now the tranny has been disassembled very neatly with each gear pack in a different tub (thanks to Hiram on YouTube) and I am looking at a rebuild kit. Thing is, without knowing why it smoked in the first place, I am worried about putting in the kit and making fresh expensive smoke. I mean, what if the issue is one of those electrical thingies in the valve body?

Can someone help me figure out why my 4L80E died so that I can make sure I have actually addressed the problem when I put it back together?

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Thank you for your time,
ɘvɒⱭ

the use of Dexron III is no longer the approved tranny fluid. 2006 GM secretly changed this to Dexron VI.

Dexron III turns to acid after 20,000 miles.