Thank you for your response, Sterling. I actually ended up finding a fix to this yesterday after some more internet searching. I followed these steps and it worked:
Step 1: Turn off iMessage Step 2: Turn on the Airplane mode. Wifi will be turned OFF automatically.
Step 3: Turn ON Wifi
Step 4: Head back to Messages and turn ON iMessage
Step 5: Now, head back to Settings and turn OFF Airplane mode.
I'll note again for anyone that might have the same issue, I had no problems with iMessage before updating to iOS 8.0.2 and it was for all messages, not just with selected contacts. I could receive iMessages, but all attempts to send iMessages resulted in text (SMS) messages being sent. The issue occurred both using WiFi and cellular data.
Thanks again!
~Caroline