Compression braking is supposed to be used to help maintain vehicle speed down long grades vs constantly applying the brakes and possibility getting into a overheating brake situation.
By coasting down a hill in neutral, you risk putting your jeep in an over speed condition. Lets say you are doing 70mph down hill, which for me with the 4.0 w/ NV3550, 4:10s and 33s, is about 2000 rpms. All of my engine accessories are running at the correct speed to maintain the proper engine environment (water pump, oil pump) and the input shaft on my transmission is also spinning at the correct rate so that proper lubrication can occur. Now lets say I coast down the hill in neutral, my engine is idling at 800rpm give or take so now my engine may not be getting the correct cooling or oil lubrication that it needs and my input shaft into the transmission is no longer spinning meaning that my transmission might not be getting the lubrication that it needs. So then lets say I coasted down the hill in neutral and towards the bottom of the hill I'm doing 75mph and I put it in 5th and let of the clutch, I'm going to risk the possibility of put a shock load on my engine, transmission and drive line components.
The rule of thumb for going down hills is that you should be in the same gear that you would need to be to go up the hill.
And if your so worried about your MPG, you probably shouldn't have bought a jeep in the first place