+1 I have a Malibu (I know, completely different animal) that has a light timing chain rattle at idle (in drive) when hot. But it's not near as complex as your 4.0; I can fix it for around $250 (my labor). I can't believe how much it would cost to change the chains on your truck..that's nuts. I'd drive it till it's dead or sell soon before the noise gets worse, as previously suggested.