I have a MortScript that someone else wrote, and I modified, that does the following, which is very similar to what you want.
If the phone is currently in 1X mode, it switches to Hybrid (enable 3G)
If the phone is currently in Hybrid mode, it terminates data connection and switches to 1X mode.
It's pretty fast (<5 seconds to run). You just run it before you get on the net, and then again when you're finished. Everything else is automatic.
Let me know if you'd like me to CAB it up