My buicks door locks work perfectly, except.

They do I believe lock after so many feet of movement. But I am not sure they unlock on their own. You might have a vehicle speed sensor problem or PCM problem with the auto door lock circuit itself for which you would have to test while moving for 12 volts to show up at the lock actuator at that certain distance. Or If you are right and they should lock in park, there should be 12V at the actuator when put into Park.