I haven't posted here in some time, but I do check in periodically. I was glad to see that Stephen Packard's thread had been active. This case still haunts me, and I still scan for any bit of information I can find. Tonight, I found the 1974 Princeton Day School Yearbook and I've posted the link to the pdf below.
Apparently, Stephen did not graduate in 1974; he is shown in a group picture of the 11th grade class (1975). It's on page 130/214, and I believe Stephen is shown in the second row from the bottom, to the right of the older man in the front row. His father and sister are pictured in the Girl's Basketball photo on page 156/214. So, after his cross-country adventure Stephen would have been returning for his senior year.
About a year ago, I did see Linda's cousin in another "Missing" forum asking that the site's owner highlight her story. I made contact with him and invited him to come to Websleuths, read the thread on Linda and Stephen and join the discussion. He was grateful for the referral, but apparently opted not to join. He seems quite a bit younger than Linda would have been, so perhaps he simply didn't have anything to add to the discussion. Still hoping friends or other members of the family might find these threads some day.
http://www.digifind-it.com/princetonday/data/pds-links/yearbooks/1974.pdf