About how much would it cost if I want to convert my 2wd to 4wd?

Ok so I have a 2003 Ford Explorer Sport Trac. Don't tell me that I should of gotten it 4x4, it was passed down to me. Anyways how much would it cost with the parts required and average of labor to convert to 4wd. Trust me if I could have chosen a 4x4 I wouldn't have thought of it twice any dumbass answers will be reported. Thank you if you take your time to help someone in doubt

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