Originally Posted by JCL

There have been various iterations of the non-OEM hitches over the years. Early ones attached to the rear sheet metal (unibody) only, without replacing the bumper struts like the OEM hitch does. There were a few problems because the hitch flexed (rotating about the attachment point). In extreme cases, this twisted the rear unibody and caused significant damage. Pictures have been posted previously on this site and others.

The response of the non-OEM hitch manufacturers was to include a strut running fore and aft from the receiver, along the vehicle centreline. This strut attaches to the rear subframe up close to the rear axle. It effectively stabilizes the hitch, but many consider it ugly, and it reduces ground clearance at the rear.

If you don't have the strut, you are probably OK towing lightweight trailers. The hitch rating isn't a guarantee, as the early hitches that twisted/rotated were all rated 6000 lbs as well.