Just to weed out the obvious...
The ATF needs to be at or greater than 150 degrees F, and
The car needs to be above 50mph in 4th, and
You should be at a steady speed, i.e., not under acceleration (or not lots of it anyway - I have no idea how much would prevent lockup), and
then you should see a drop of about 200-300 rpm (at around 50mph) - that is the TC locking up.
BTW, I have an aftermarket cooler and it takes me about 15 miles of driving, in Florida temps, before the fluid warms up enough to get lockup.