Hello,
I would recommend you replace both arms (is it the arm that connects the rear wheels to the chassis? )
I recently suffered a break in one of the arms on my 55 plate Mondeo (fortunately at 5mph in a B & Q car-park).
I haven't removed the broken arm yet but evidence shows that the arm must have been damaged ages ago and it has corroded through, enough to cause a fracture)
Andy
September 2008