Similar issue. This worked for me on Win 7.
Uninstall the LG Drivers and reinstall the G2x 2.2 package from LG.COM /us/support/mc-support/mobile-phone-support.jsp (I can't post links)
Plug the phone in and go to Device Manager. There should be a USB Composite Device that is not properly installed. Right click it and select Update Driver Software.
Select 'Browse my computer', then 'Let me pick'. Choose have disk, and browse to where the LG Drivers are installed. Likely C:\Program Files\LG Electronics\LG United Mobile Driver\ and select lgbus9x.inf.
Then pick LGE Android Platform Composite USB Device from the list of drivers. Should be the top option. Hit next.
I let it sit for a minute while the drivers installed. Some things registered in device manager right away, the LGE USB Modem for one. But, the drives didn't. Unplugged the phone, plugged it back in, and the drives popped up under Portable Devices.