I have a tuner and have built a transmission. In doing both of these, I've asked many questions and learned quite a bit from transmission guys, tuners etc etc.
Our transmissions will and can lock when the shifter is in the 3rd gear gate (no matter how it'* called 3 or D with 4th being OD). The tables in the tuner specify that the trans be in third gear and above a certain MPH with certain throttle percentages. This means that TCC won't lock when you are under say 70%+ throttle. I have also done many tests as I've had my share of issues... and been able to get lockup with the selector in 3rd. If the TCC is staying locked you should see a code for it. I think it'* P0742 TCC stuck on. Not sure I'd give the generalists at O'Reilly'* too much, they may be great guys..but they aren't quite trans specialists.
To my knowledge I have never seen any programming regarding the downshifting. Which then leads us toward a seal or maybe the trans not shifting down out of 4th properly.
4th gear has only two friction plates where most other gears have around 7-10ish. I forget the actual number. This can heat up the 4th clutches and cone them (shape wise) due to heat. It could also potentially adhere them together.
00 Regal GS: All kinds of cool stuff.