My 05, 6.3 litre is in the tranny shop. They told me to be in Tow/Haul mode whenever I am towing my 12k pound fifth wheel. I can leave it in tow/haul but when I level out from a hard pull it seems to shift back into what I call overdrive. The tech said the torque converter is still in lockup. So whats doing the shifting? Is he right?

I would sure like the real scoop and this has always been the place to get it.

 

6.3L?

T/H is just a different shift and lockup pattern. It's is not overdrive lockout, so activating T/H will not prevent the truck from shifting to 4th.

In T/H mode, the torque converter will lockup in 3rd and 4th. Usually (at least on mine) the converter locks right away when shifting to 4th. It may feel like one event when it happens, but it's really two: the overdrive clutches engage in the rear of the transmission in the overdrive unit, then the torque converter lockup clutch engages in thr front of the transmission in the torque converter.

 

There is a flash that will disable 4th gear overdrive when towing.
https://www.dieseltruckresource.com/....html?t=225627