Why do f-150's use IFS but the f-250 and f-350 use a solid axle?

Is this the case, and if so why?

Thanks,
Neil at the Jersey Shore

 

As far as i know the reason they use solid front axles on the superduties is for strenth. Someone correct me if im wrong
John
99 f-150 4x4 ext. cab(84k)
rear steel buper..grilleguard w/4 KC lights
tool box..bed linner
dual exaust..K&N air intake...<!-- / message --><!-- / message --><!-- / message --><!-- / message --><!-- / message -->

 

I would agree with SBEII.... IFS is good for handling and ride, but solid axle is stronger. With the load capacity going up, they went to solid for that.