If your automatic transmission is not shifting and you have other issues, it quite possibly be the neutral safety switch and it is located on the driver's side of your transmission behind the shift lever. And if you have to replace it use a hair dryer or a heat gun because GM glues the wiring harness plugs on to it and they are a pain to remove. You quite possibly may have to replace the plugs. This is what one looks like.

 

Good advice!