We just all assume that the frame rails were straight when the frame was assembled. However if you do remove the frame please sight down the lenght of the top and bottom of the frame. I am curious if the drivers side frame rail was bent inward just forward of the cross member during assembly and thus cause that deflection at the center of the crack.
Also was the lower flange of the frame ever welded to the lower flange of the cross member or just along the vertical seam. The picture doesn't seam to show any welding on the inside but it may have been welded along the bottom.
This is what I found on the receiver when I returned from Alaska. Not sure I can blame Alaska but that when I looked.
WBCCI 12156 AIR 3144 WACHUNG TAC NJ6
2004 Excursion 4x4
1991 34 ft. Excella +220,000 miles, new laminated flooring, new upholstery, new 3200 lbs axles