I may have solved my problem with the TCC locking up at 74MPH @ W.O.T, while tweaking some of the other TCC settings for my van today I found "Kickdown pattern TCC engage/release" and in "Engage MPH vs gear" 3rd gear factory was set to 75MPH.
I changed it up to 125.5 and for the first time I got to 78MPH @ W.O.T without the TCC engaging. Typically I had two cars in the lanes in front of me so I had to lift off before I could I get a higher MPH reading!