I have a 1991 2wd ford f250 auto transmission xlt lariat 7.5L v8 is it possible to make it 4wd what would be easiest cheapest way to do it?

I have a 1991 2wd ford f250 auto transmission xlt lariat 7.5L v8 is it possible to make it 4wd what would be easiest cheapest way to do it?

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