What year is your truck? Like a 2001-2003?
My 01 has the timing chain rattle caused by the front two timing chains. However there is a timing chain in the back as well. And just recently (when cold) I have a rear timing chain noise as well.
But to answer your question, the fronts can be replaced kind of easily; however to my knowledge if the rear one ever fails the entire engine has to come out. Ford had a TSB out a while back and has since expired where they would replace the front timing chain components for free and did nothing to back timing chain.
I have been running my truck for 7 years (I bought it in 05 and now sits at 130 on the odo) and it's had the timing chain rattle ever since then and I have never had any problems. So I'd just run the engine until it the chain finally breaks/falls off then replace all three cause replacing the front timing chains is a lot easier done when the engine is out of the truck as well.
-2011 F-150 Lariat: 5.0 "Coyote" V8, 6-spd Auto (6R80E), 3.73 ELocker. White Platinum Metallic Tri-coat.
-1992 Ford Ranger: 4.0L OHV, 5spd (M5OD-R1), 3.27 Open.