I had the same problem. I have done research on the web and found that one problem may be a pin coming loose in the shifter assembly that activates the micro switch that sends a signal to the ignition. This pin, about 3 to 4 inches long, has two uses; 1) to prevent the shifter from moving out of park without the brake pedal being pressed and 2) to allow the key to be turned off fully.
I had both issues above so with me being cheap and not wanting to spend $200 - $300 on a new shifter assembly I did some investigating. I took apart the center console and discovered that the pin did in fact rattle lose. The pin was laying along side the left side of the shifter assembly I took out the four nuts that held the shifter assembly in place and located where the pin came from. The pin can be reinserted about mid way on the assembly just below the micro switch which is located on the right hand side of the assembly. The pin can be kept in place with what is called "Lock Tight", a glue like substance that will stop the pin from rattling lose again - Just put some "Lock Tight" on the grooved portion of the pin. You can buy a tube for $4.00 on Amazon. Total cost to fix = $4.80 (including shipping). No need to buy a new shifter assembly or take it to a mechanic and pay them to fix it.
Hopefully this helps out!!!