So, to clarify.
You have a custom rom right now, that you want to keep.
You want to upgrade you're radio and then flash the custom rom again, but you also want to restore a backup because you've customized a bunch of stuff after the rom was loaded.
Correct?
If that's the case, you shouldn't have a problem.
Just upgrade the radio, but be sure to follow the directions on how to do that.
Then reload your custom rom and restore your backup.
Backup's do not affect the radio.
After flashing the sprint rom with the 3.37.15 radio, I need to reflash my custom rom and can then run my backup to restore all my settings? I guess I can't just run the backup since the radio upgrade is more than just the radio?
After flashing the sprint rom with the 3.37.15 radio, I need to reflash my custom rom and can then run my backup to restore all my settings? I guess I can't just run the backup since the radio upgrade is more than just the radio?
Correct.
Flash the rom to upgrade the radio.
Re-flash your custom, then load your backup.