How can I defrag a large RAID?

What's happening when you try the native utility?
Anyway, you can use Diskeeper 2009 Pro; it should be able to do the job easily. It's specs include support for arrays and mirrored volumes.
http://www.diskeeper.com/diskeeper/home/protechspecs.aspx
http://www.diskeeper.com/diskeeper/home/premiertechspecs.aspx
You can grab free trialware from the above links.

Are you experiencing any problems, performance or otherwise? Is it a hardware RAID or software RAID?

If hardware RAID, the RAID controller has it's own algorithm's for storing and optimizing data on the volumes and defrag is likely not necessary nor advised.