My car does that, and I think it's 'cause of the bad guide rod mounts...the part of the front suspension that goes from the lower control arm back to the subframe. Mine are bad enough to allow the two front wheels to move forward and back about 4 inches...
If you want to check this, put a lug wrench on one of the wheel lugs and try to turn it...if you can get the whole wheel to move (without the rest of the car moving), you've found at least part of the problem.
Just a thought...good luck!
{on edit} Also, if you're not setting the parking brake when you put it in park, the car'll move like that...and IMHO it's a real bad idea to not use the parking brake. I for one don't want to trust a small gearset and a pin to keep a 4000lb car from moving.