I did a quick search. It looks like another young girl was dumped on I45 on Dec 10, 1980 - this was farther south, but north of Houston. There's not really a description of what happened other than "homicide".
While searching I also found a PM photo of WCJD that I have not seen before. There are numerous PM photos on this page, so please don't click if you aren't comfortable:
http://www.harriscountytx.gov/ifs/UnidentifiedDecedentFliers.aspx#J1989
You will have to scroll down to the 11/1/80 entry to see the picture which is in color, contrary to the other 2 I have seen. It appears the information is from the Harris County ME office. Note, the document filed on the page says she was found 4.5 miles north of Huntsville. That correlates to the 1696 exit and not the culvert a mile from the hitching post. I really don't feel like the latter location and description should be given much weight any more.