Well...Im not an LDAP expert but I couldnt get the query line you posted to return anything. There was also a missing "\" in the drive mapping statement. I was able to get the below code to work in my env....Give it a shot.
Code:
$user_dep=GetObject("LDAP://CN="+@FullName+",OU=users,DC=aaa,DC=com")
use g: "\\server\"+$user_dep.department+"$"
