"The airframe was not designed to handle the load from the winglets...the winglets came later..."
The outer wing was modified with different structure parts before the winglets were installed on the post factory retro-fit NG aircraft.
This was strengthening for the wingtip and replacement of the spar with one with boltholes for the winglet...the only stiffened element was the spar third in from the wingtip...
The rest of the wing and the pickle fork connector between the winbox and fuselage remained unchanged.
The AD applies to all N-registered 737NGs. It doesn't differentiate between passenger and cargo aircraft.
Thanks...it seems odd the cracks (so far) have only been found on the pax version....