I do not know any about your great grandparents but I have two books here with helpful information for you, both printed in the USA.
Ancestors in in German Archives, a Guide to Family History Sources, ISBN 0-8063-1747-7
Page: 453, City and Regional Archives:
Bad Doberan, State of Mecklenburg-Vorpommern
County Archive address:
Kreisarchiv Bad Doberan
August-Bebel-Str. 3
D-18209 Bad Doberan
Germany
Website: www.lk-dbr.de
E-mail address: kv@lk-dbr.de
They say in the book they do not have no emigration records, but I think they should be able to guide you to the right addresses.
The other book, a photoreprint of an old German book, printed in 1912:
Meyers Orts- und Verkehrslexikon des Deutschen Reichs,
ISBN 0-8063-1634-9 (volume 1); the book
list all the names places in that time in Germany!
Page 751:
#48, a farm place named Heide, to find that look for Buchholz, District Schwaan, Mechlenburg-Vorpommern.
To locate all of this look at first for the city of Rostock, west of the city you will find Bad Doberan, then go south and you will find Buchholz and more south Schwaan.
Hope this will help you,
Wilhelm