Swaps are easy to do, if you have the right tools. The automatic tranny in the GC is the same as the one in the XJ 4.0. I have heard stories of people swapping in the 304ci, the 318 or the 360 into their Jeeps, and using the stock automatic tranny.
How hard is it? Well, if you want to continue using fuel-injection there are computer issues to look at (that's the biggest issue), the motor mounts shouldn't be too hard (call up the dealership and get stock V8 motor mounts) .. and then spend some time working out the procedures for the time required to do the swap. If things go well, you would be done in under a week ... if it doesn't go well, you could be lookin' at a year