The bracket shouldn't "walk" on the frame if properly installed. The bolts which clamp the bracket to the frame are supposed to be torqued to 65 ft-lbs.
You level the brackets when installing them to the frame, before fully tightening the bolts. They can be tilted back and forth slightly to align things. Just get everything snug, then use a straight edge or yardstick resting on the "L" like the bar would to make sure the flat points to a couple of inches below the bottom of the bars on the hitch head (because the tongue will come up a couple of inches when the bars are in use). After they're aligned, go back and torque the brackets down.
