I have a 2011 stx 2wd and I'm planning on putting a 2.5" rough country leveling kit on it along with 33x12.5 mud graps. I want to put 20x10 rims with -24mm offset on it but I don't know if it will rub any? Or should I go with 18x9 rims with -12mm offset

 

The only chance you have to fit that wide of a tire is to keep stock rims but they are too narrow for the tires and will still likely rub the steering knuckle/upper control arm.

-24 and -12offset are way to much for a 2wd to fit those wide tires without rubbing. A lot of trimming would be required.

You will need at least an 8.5" wide wheel and about +25mm offset to have a prayer of fitting 33x12.50