Firstly traction control and ESP ARE NOT the same thing. they are entirely different and operate independantly.

AS mine is a SAAB and so GM like yours, it could be that although you can turn off the traction control you cannot turn off the ESP.

I too have been stuck in snow on a slope - turning the TC off doesn't help. I have used strips of old carpet to get me going in the past. Use some rope to tie them to the car and drag them behind for later re-use. You can't fit snow chains to my tyre size so I have tied carpet around the tyre, chain-style - surprisingly effective.

Winter tyres would be even better.