This worked for me ... you must re-enter if you reboot or if you enter airplane mode, but that is not a big deal to me ..
Originally Posted by BecomingxInsane
I found out that if you go into the dialer and type in *#*#197328640#*#* that it will take you into a menu that will allow you to change it. From the main menu of the ServiceMode screen, select DEBUG SCREEN, then PHONE CONTROL at the bottom, then UE STATE CONTROL. From there select CHANGE RAT TO WCDMA to enable 3G only, and select CHANGE RAT TO GSM for 2G only.