As far i can see in my W2003 AD domain (can't check W2008 from here), groups don't have a displayName attribute (while users certainly do). So that would explay why you get an empty value for that attribute.
Regarding the 'Update local data" and "Lock value" settings, they have never worked (the underlying code was never implemented) and have been finally removed in Moodle 2.0 and later versions.
LDAP Enrolment AD attribute mapping
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