Did you know!!

[Goats]

How hard it is to find a 3rd gen without a rusty undercarriage or a corroded rear hatch down in south Florida. Every one I went to go look at that's a 4x4 came from up north.

 

[american steel]

my 2010 navigator is in Delaware we have snow and salt problems here but my navigator doesn't have salt or corrosion issues underneath or even the hatch.
you living in florida you have a problem with salt water in the air on both sides of florida are surrounded by salt water, so i can see that being a probable cause.
just like the beaches up here a lot of these vehicles are rusted too for those who live near the beaches.
you might not be looking at northern expeditions.

 

[Goats]

My 03 doesn't have the rusty issue under neath

 

[american steel]

My 03 doesn't have the rusty issue under neath

I must have read your post wrong.
after looking at it I don't think it should be that hard but you must remember there are a lot of vehicles that have rust and corrosion issues when they're living beaches and salt water areas.
my aunt lives near the beaches of san Diego she has a 04 expedition she bought when new it has rust underneath of it.
her hood had to be repainted also. under factory warranty ford had to replace the hood after aluminum corrosion problem a year after the hood was painted.