Welcome to GMForum!! This thread is a bit old, and in the future, please start a new thread. Just go to mechanical and maintenance for your year/vehicle and start. If your post is misplaced, the moderators will move it.
So to clarify, you have your car in "D" or "3" and it will not move. If you switch to "2" or "1", the you are able to accelerate and shift into "3" and then "D", as the car is now up to the correct speed? I admit i am just learning about transmissions, but I will send one of the moderators a message, and he can grab your post.
Again, welcome to GM Forum Darrell.