Well, the way I understand it, is that 4th is the overdrive gear and will downshift more frequently when in the overdrive gear selection.
I've seen many of the reasons about not towing in overdrive. I think, (and don't hold me to this) that the only real difference in the tow/haul is that it changes the shift points and does not allow as much hunting as a vehicle that does not have the tow/haul mode. That said, the truck still tows in overdrive, it's just modified by the tow/haul button I believe.
At any rate, were taking it out on a few shake downs soon and will get about 80 or 90 miles each trip of real world input.
I'll let you know if my poor 4L60E can hang with it's more robust 4L80E brother.
I sure hope so, as I have sort of bet the farm that this car will deal with the Safari for 3 or 4 years (hopefully without a bunch of trans rebuilds)!
Eric