I'm not familiar with the F150 other than I learned to drive in one with a granny 1st and four on the floor. If it uses a Saginaw steering box like MOPAR trucks there is an adjustment on the top to adjust the height of the pitman shaft inside the box. If it's never been adjusted it might be worn out by now but the adjustment is easy.
Try this one out.
http://www.monte-list.nu/tech/boxmesh.shtml
or this
http://jeep.off-road.com/jeep/article/articleDetail.jsp?id=259494
Here's one about a 1993 F150 with sloppy steering
http://autorepair.about.com/library/faqs/bl136h.htm