I think it was called a stake pocket tie down or something like that.
I got mine many years ago at wallyworld & paid way too much, but I've seen them at harbor freight. Not sure how strong or durable the HF parts are, but mine are over 10 years old & still holding up nicely.
i got some from harbor freight that were rated and 2000 pounds. i hated them they wouldnt screw down all the way and rattled as i went down the road. i would get these http://www.autoanything.com/hitch-bed-accessories/73A4436A0A0.aspx even though they are expensive they look like great quality.