I have a 09 Chevy Tahoe and it is stuck in
drive and the only way to keep it from rolling
away is to put the emergency parks on
Gear selector switch is the first thing that pops into mind, it maybe stuck in drive setting. A live data scan tool can show you the position of the switch. it could also be an internal transmission problem, with all the electronics that control stuff it is really hard to say just exactly what it is.