Based on the pics the frame looks like it has a fair amount of surface rust. Definitely look up and down the frame and at the rear upper shock brace. The welds on each end are known for cracking. As said above, under the doors, pillars, floors, body mounts, rockers.
Since it looks like the truck was resprayed at some point, cruise the body and knock along to listen for bondo at the wheel arches, around the windows and any bubbling. Also look for bubbling around the roof gutters.
EDIT: Just realized this was DC/Metro area, makes sense why it has such a dry frame and surface rust. Has it lived there its whole life?