Taco
I will have to check but I don't think the 6L90E will stay locked up at 5 or 10 mph in first gear. With your foot hard on the brake. As I said the turns I am talking about are at a near stop. Even in granny gear I am still on the brake first because it needs a little brake. Second just for reassurance.
I am not familiar with the tow haul modes, but so far I have heard about 60/40 that they don't hold in the situation above.
I had hoped the new Auto's were better than my brothers late 90's Ram with a after market lockup switch. Even that wouldn't hold lockup through the hairpin turns. And when that came un locked in the turn the first time. It didn't send you home smiling.