One timing chain is in the back on the 4.0L SOHC because they use identical heads on both sides. If you look at the back of the driver's side head under the intake assembly, you will see that it is fully machined and threaded for a thermostat housing, and is merely closed up with a freeze plug. Saves a bunch of money making it if you have only one line making one part, rather than side-specific castings.