[CJ3FLYR]

It's frame flex and cab 'float'. The cab is rubber mounted and sits as a basically rigid unit on the frame, the bed is solidly bolted to the frame. The flex occurs relatively evenly along the frame but the only place we can see it is the crack between bed and cab. This is because we don’t perceive the slight flexing of the bed/aft section of the frame and the cab rides as [almost] a separate unit floating on the front section of the frame. Sounds bad but it's fine and if they eliminated it completely our trucks would weigh too much, be too slow and use too much gas. In truth our frames are amongst the stiffest in the 1/2ton segment.
If you go for a ride in a current, empty, 3/4 or 1 ton from Chevy or Dodge (the Ford chassis is very old and does flex a bit more than Chevy or Dodge) you'll see they do it to a much smaller degree. The frame of a one ton needs to be incredibly stiff in order to deal with the 4000+/-lbs payloads those trucks will handle in the bed. Our trucks don't have that kind of capacity and don't need that last bit of rigidity.