Wow, just finished fixing this problem last Friday!!! Also Google TKO 600 stuck in gear and you will find lots of info on the WWW. The Chevelle forum had the best info.
I took my car to the Big Bend Open Road Race a few weeks ago and on the third practice run my TKO600 stuck in 4th gear. Yes there is a quick fix but will reappear. I had Rosehill Performance here in Houston take the trans apart and fix it. He found the shift fork bumpers, little plastic things, were destroyed from shifting hard. He replaced them with bronze ones and then removed and rebuilt one of the shift gates that has to be beveled a certain way. $400.00 later it is BETTER than new. After hours of reading on the internet this problem seems to be with tremecs produced around late 2006 and early 2007. The TKO 600 was new at that time. I personally know of three Cobra's that have had this problem. Seems like alot in our small world.
Try reading this thread;
New TKO 600 stuck in gear? - Chevelle Tech
And you will find lots more info. Read it thru and all the threads associated with it and I bet you have a better understanding.
Larry Boatner