Try this thread. Look at the verzion radios there and download them. Sometimes, also, flashing back to stock helps reset the phones system, and uploading the new rom and radio might work better.
CDMA Diamond Radios
I am using an alltel diamond as well, but it is "powered" by the verizon network...meaning that all my stuff is now with verizon instead of alltel i guess. I just stick with the latest alltel radio, i get awesome signal. Also, it depends on the location where your at.