I had this same issue when I first got my hero. I decided to call up Sprint, and it seemed as though they've had calls about this problem before because they told me to follow a few steps and after about 10 minutes text messaging seemed to be normal. I suggest just calling up Sprint and seeing what they say.
Interesting.. I didn't even know Hero had the option to choose, that's pretty cool though, I kept mine on Hybrid though, but I changed NAM settings from NAM-1 to Auto-NAM, not sure what this'll do, but auto is always better than manual in my opinion, lol