Here is a link explaining more and a chart to check out if you wan't...
http://www.sporttruck.com/techarticles/1392/index1.html

They say that I beams may be the best thing Ford has ever made for their vehicles... If you ask me I hate em. Maybe they can start making 5" dropped I beams or so and have the alignment all nice and I will start liking them but until then I don't like them.