You need Mechanial Desktop or Inventor, is the short answer. I believe that there may be an IGES plugin still available somewhere, but I can't point you to it.
AutoCad is basically a 2D only system. The safest bet, is to find someone who has Mechanical Desktop, (a 2.5D system) or Inventor, (true 3D) and convert it through one of those packages. Otherwise, rebuild the data from the prints.