There are many variations out there...the important thing is that it is strong enough to support the bed and that it locks in-place so it cannot get knocked out. That's the biggest problem with lumber...it may have a weakness that is unknown and it can get knocked out of the way.
Here is a link to one I found online that looks like a good design. Also in the picture is a commercial removable type. You could also make something like that and not have to add anything to the truck.
http://www.osha.gov/dts/shib/shib091806_fig4.jpgIt looks like the rod is flipped up and held in the correct orientation to catch the upper angle by the piece at the bottom. Raise bed, flip up arm and lower bed. The arm could be flipped up with a stick so that you never have to go under the bed.
You might also want to think about putting one on both sides. Extra safety never hurts. Depending upon clearances you may even be able to tie them together so they both raise and lower together.
Tad
