Okay, this is going to be difficult out of the box without motion capture, because you are wanting to rotate and translate the root node of your figure's skeleton without affecting other parts. The crudest way is to note the XYZ trans and rot values of the parts directly above and below the hip, start keyframing one thrust cycle, and manually correcting each part as needed. The upper body should have little problem with the action, as the business is in the hips and legs. Unless there has been new IK solver added to P11, then IK is an all or nothing condition; toggling it will only destroy your animation. If you are using IK on the feet, then all you need to do is make sure the bends in the legs are sufficient to keep the feet flat on the floor. Aside from keeping the upper body from swaying and bobbing around (which should only need correction of the stomach/belly trans and rot values, since nothing more is moved above the hips), the bulk of the work will be the hip, thighs, knees, and ankles. Use limits, and you won't have to worry about joints getting too badly out of whack.

Once you have one complete cycle, all you would have to do is cut/paste that cycle onto the dopesheet, and you can make as many thrusts as you want. If anyone knows a shortcut I've missed, please feel free to jump in. I had day surgery thursday and am still not hitting on all cylinders..... :P