The '12's all have the 6R80 6speed, with 5th gear being your 1:1 and 6th as OD. (Well, 5th is not really a true 1:1 gear ratio, but very close to it.)

If I remember correctly, turning of OD will only lock out 6th gear in the 6R80. Unless 5th gear is also considered OD.

 

I used to play with manually shifting until it got stuck in 3rd gear. No matter what gear I set it to, it would not shift. Kinda sucks when you're on a road speed limit of 55 and cars normally going 65 and I'm trying to keep RPM's steady going 40-45. Had to turn off the road, shut the car off and it reset itself.

Like everything electrical, gliches happen but I wasn't too thrilled about it. Glad it wasn't a true highway. Oddly enough, it was a 3 lane road and a Mercedes was randomly following me behind while the other lanes were open...