Not hard if you can wrench. Some will tell you to get rid of that jeep and just buy one that already has the 4L in it. I would have to agree. I put a 350 in mine and it wasn't that hard. Just took a while since i did it by myself.
i put a 4cyl into a frame that had the 4.2 I6 in it, it was easy because i was swaping everything..i did have to cut and break the motor mount welds on the frame and relocate them. I used imaginary lines off of another parts jeep i had that had a 4cyl in it. my recommendation is if you decide to do that, relocate them and tac weld them then put your motor and trans and tcase in to make sure it all turn out right. but keep the cherry picker hooked on the motor for support. once its all in place and works good, then finish welding in the mounts