my phone hit a pavement and 3g/4g stopped working. next step?
In the HTC Dream (G1) the UMTS radio (3G) is a separate board from the rest of the phone. While this is a Desire Z and I haven't had the opportunity to take one apart, it's probably very similar.

The 3G/4G daughterboard (or if it's connected, antenna) could have failed, cracked, disconnected or sustained damage in some other way. Software won't fix it. You have damaged hardware.