If you mean the welded seam where the floor pan and side body panel meet,
the same thing happened to mine.  I think I was the culprit, though this was
many years ago.  When it started to rust, I finally just went after it with
some pliers to straighten it out, cleaned/scraped/prepped it good, and gave
it a good coating with POR-15.  Have had no problems since, and it looks
fine from 6 feet away.

However, don't jack from there in the future.  With that seam trashed, the
structural integrity on mine was gone, and I ended up bending the sheet
metal the next time I jacked it up.  And that I have not fixed - way beyond
my limited ability.