Hi Merrie,
I mentioned the harness system I use for Trevor in the crate thread. Of couse he is in the cab with us, but check out these links from the manufacturer's site for ideas for securing the restraint strap in a pickup bed:
Champion Canine Seat Belt System
CHAMPION CanineTruck Restraint
That carabiner-type hardware allows you to connect the restraint strap to just about anything, including tie-down points in a truck bed.