coolant leak

My 99 4.0L was leaving a little coolant on the ground so I took it in for a pressure test expecting to need a new pump (the truck has 170000km/100000mi). My mechanic who I trust said that the timing chain cover was the problem, it has a crack or a wear through or somthing, he said that the cause is a timing chain that is on the way out. The cost is a little steep but I will do it (new chain kit, new cover, new pump). I haven't heard of any 4.0l having any chain problems, what do you guys think? Does this sound leggit? Is it likely that more damage is to follow from the metal chips/coolant contaminating the oil? Has anyone else had this problem and how did it turn out?
Thanks Guys.