You don't need to unlock to flash a radio. Check out the thread in the upgrades forum, there are radios in there that you don't need to be unlocked for and it's just like flashing a ROM, you can even do it from SD. But it's gonna be trial and error, I've tried a couple but end up coming back to Sprint stock as it seems to work the best for me but YMMV.
Here's the link to X's thread.
http://forum.ppcgeeks.com/showthread.php?t=90342