If you are using 2″ galvenised nails and the wood is old then the point of the nail needs to be flattened to crush the wood on impact rather than separate the grains in to a split. Just turn each nail point up on a concrete base and give the point a tap with a hammer to take the point off. Alternatively use clasps.
This is the easy way to remove fence posts and concrete. Contact your local hire shop to see if they have them available