Check for play in the slip joint by holding the lower section and moving the steering wheel side to side. If there is play in the joint I usually remove the slip shafts (requires loosening the u-joint under the front trunk) and take a torch to them and beat them back into submission. I don't favor the cotter pin approach as if it is not an exact fit in the hole you haven't solved the problem. Some folks put a little tack weld between the two, but that renders the collapsible function non functional.
There are also plastic bushings in the top and bottom of the steering column sleeve, these often fail and allow some lower movement, replacement bronze bushings are available.