Contact the USG company. They may be able to give you more info.
USG 125 Franklin St. Chicago, IL 06060
I believe that asbestos was used for a short time in drywall. Though I know about as much as Steve does. As he has said, always wear a mask. You can never be too careful. As much a worry is Sillicosis, as hardening of lung tissue due to the inhalation of Silica dust, present in lots of contruction materials, from Varnish to joint compound.