I had a 99 GT (selling it tonight! just got a 08 GXP) with the same issue - no warranty either. Apparently when you put the car in Park, an electrical signal was not being sent to the cylinder in the ignition, thus not allowing you to turn the key all the way to off and remove it. Until I was able to get it to the shop and have them determine this, I also had to leave the key in the ignition, which would drain the battery DEAD. They fixed the wiring (for at least $150+) and everything was fine. 8 mo. later it happened again. I was not paying to have it fixed again. I should say that they weren't able to fix it right away as the guy that knew the electrical on that part of the car was on vacation. Until he returned, they gave me a little screwdriver and showed me the trick. I simply reverted back to that when it happened again. For the last year or two, that's what I did... every time. Pretty soon, I didn't even notice anymore.
I bet that doesn't help much, but maybe the background info will help out someone more capable of the fix than I.
good luck