Help me understand. Series 2 XBees either broadcast or unicast. The difference is that unicast is routed to only the address you specify and broadcast goes to all of them. If I understand you correctly, you've been testing using unicast and now you want to broadcast instead, right?
If I got it correctly, then you may be suffering from data timeout. Take a look at the XBee document under unicast timeout and try out the numbers you're using. In unicast mode (point to point transmissions) the XBee will automatically extend the packet timeout so the end device can have a chance to receive it. In broadcast (one to many), this doesn't happen. Also, in unicast mode, you can look at the transmit response code to see if the message actually got sent correctly and you can require an ack to tell you the other end got it. This stuff doesn't work the same in broadcast. In broadcast, you'll get a good transmit response code if it can send the message at all and the ack works differently. Assuming you're using API mode, not AT mode. In AT mode, you don't get any of this stuff.
The XBee document: http://ftp1.digi.com/support/documentation/90000976_G.pdf
and they talk about timeouts down in the mid 60s somewhere.