With PowerPose you can activate the translate gizmo, which is a set of axes you can drag to move in just the desired direction, or squares between the axes you can drag to move in a plane (some graphics cards are able to run D|S but don't let you use the manipulation gozmos, however). That won't completely remove unexpected translations in tha nother plane - the parts are rotating about their joints, so if you drag past the point where everything is at full stretch something will have to give - but I find it works farly well, to the point where I am willing to use PowerPose as I wasn't to use the in viewport tools in Poser. However, I wouldn't try doint very fine posing with it - my brain seems to be wired wrong.