the problem is the emissions. When you switch the newer engine into your older jeep and try running it with the old computer it will not run as it should. You have the o2 sensor,coolant temp sensor and many more sensors sending info to the computer so it can adjust how much fuel it need and so on. if you run the old computer it may give the wrong info to the engine which may cause poor fuel milage,and other problems.
Another problem is if the older connectors will even hook up to the newer connectors. they change things to make them work better or to fix old problems. that is why you should switch the entire wiring harness and computer.

If you switch the front end it will be a direct bolt in.The 2 wheel drive front has all the springs and such in the exact same place as the 4 wheel drive. the trans and transfer case will bolt straight in as well. I am willing to bet you could switch the entire running gear (front diff,trans,transfer case,rear diff and driveshafts) quicker than you can switch out the engine and wiring.