Taking what you said at face value, that you are getting 4" groups at 20 yards and pieplate groups at 38 yards, and your groups at 20 yards at within 2" of where you want them, I wouldn't think there is anything wrong with your form. Not that we all can't improve our form, however well we shoot, but I don't think a form change is the solution to your problem.
Try using 145 grain points, try shooting a longer arrow, and just shoot a lot, and I'm sure you'll be on target pretty soon.