I've seen the factory timing chain go 250K plus in 2.9 engines
The very similar timing chain setup in the 4.0 has a better tensioner design and the chain is under less stress, because to spin the cam in a 4.0 takes only 30% as much force as the one in the 2.9.
Sound strange? Two word for you: "Roller Cam"
There is NO reason the timing chain in a 4.0OHV to EVER wear out.
AD