1999 Ford Explorer Limited

I have a 1999 Ford Explorer limited with power door locks. They automaticaly lock when I start to drive but they don't automaticaly unlock when I put the vehicle in park. Is there anyway to change this?
If so how? I love the SUV but this problem really gets on my last nerve. I am constantly driving very important people around and I can only imangine there fear when we stop and they can't exit the vehicle from the back doors because they are locked in. Any help on this would be greatly appreciated.
Thanks, Hopalong

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