I had the same problem and it's dead simple with the correct tool. Check the type of head that is on the bolt holding down the receiver on the body. If its a hex head a small socket will do but some are star heads.
Simply slacken off both bolts, move the receiver towards the rear and re-tighten. You may need to re-position once or twice but it will take no more than 5 mins.
Ray, August 2007