Hi Jeff -
GMAC is now known as Ally Financial. Contact their mortgage branch at 1-855-256-2559 or visit their web site at www.ally.com.
Many GMAC mortgages were turned over to the new DiTech company for servicing, but I don't know if they'd have information about older loans. You can contact them at 1-800-700-9212 or visit www.ditech.com and search for "GMAC."