yes use a UNC share.  The server you are backing up to - is that on the same domain as the DB server.  Is your SQL Server running under a domain account?  If your SQL Server is running under a domain user account and the server that you are backing up to is on the same domain, then you should be good.  If the server you are backing up to is not on the domain,  then you can create a local account with the same username/pw as the account SQL server is running in and give the account the neccesaary permissions on that server and it should work fine.