I am in the middle of moving, so it will take me a bit to find the code. However, the main issue turned out to be that I had wired the RS-232 to TTL converter incorrectly (see above). You can't just connect the Netduino directly to the sonar for serial data transfer.
For the serial port, I had the problem mostly solved by just reading 5 bytes and discarding them if it didn't read correctly, but then my Create developed a problem sending serial data, so I switched to the sonar analog output--I haven't had a chance to get back to the serial transfer yet. Important to remember that the Netduino analog input will NOT tolerate 5V--you have to use 3.3V.