Hi luvtheus,
Here is some good information about pick up truck aerodynamics. It's a study done by Feysal Ahmed Adem at California State University.
http://csus-dspace.calstate.edu/xmlu...pdf?sequence=1
The most useful information is at the bottom of the report. There is a good picture showing flow with the tailgate up and how the air interacts with the bed, tailgate, etc. There are lots of good diagrams in that report. I'll have to thank the guy who first posted it to ecomodder.
And just as Frank Lee said short answer, up is better than down.