I too have found that standing up, off the saddle causes big sway issues. I modify my riding style when doing loaded touring so that I don't stand up. If I need a little additional power, I may rise slightly out of my saddle, just a couple of inches, for short stretches. But I never stand up when climbing hills, which slows me down slightly, but it's not a huge problem. If you like to ride out of the saddle for long stretches, a bob trailer would be something worth looking into.