I guess I didnt consider that the broken chain link could be on the road. I did look all over the road around where I unhitched and didnt see anything, but that doesn't mean it couldn't have snapped off and flew into the ditch or something. It just makes me nervous that a chain and hook can fail at such a low speed, especially without any apparent damage to either the chain or the hook. The hook was left clipped to my truck. I defintely learned a lesson from this: Always jack the trailer back up to see if it lifts the truck! Thanks Ben, I think I will be upgrading my hooks to something a little more sturdier.
Here is what my chain and clip look like: (Hopefully image comes over this time)
http://www.towsmart.us/products/100-epdm-rubber-4-x-17-5-smartstep-w-adhesive-backing-1-pk/