Win2k Domain Consolidation

Try Technet article Q238394 on MOVETREE.EXE, which can move objects between AD domains. So, you could move all users and computer accounts into the single AD domain, then run DCPROMO on the other Domain Controllers to remove them from their other domains, then add them into the single domain.
Here is the URL to the article:
http://support.microsoft.com/search/preview.aspx?scid=kb;en-us;Q238394
(please remove any spaces)

hope this helps