Ever wheeled an XJ for long? They break.
If abused enough, everything will break :tongue:
Some simple upgrades make them pretty solid up to 35" tires, and they will easily outflex an IFS tacoma beer
Yes i have personal experience with this stuff, I wheel a ZJ (Grand Cherokee) on 35's.
I'm not sure why you hate 4.0's. I've had good luck with mine, and so have many others. Mine is underpowered, but that can be fixed. i'm going to swap in a 360/46RE, but only because they came in my Durango. Otherwise i had plans to stroke it to 4.6 and put in an AW4.
When it comes to Unibody's they can take some abuse, but when they're done they're done, gotta start over, at least that's the easiest route most of the time. With a frame, if it goes you can either buy a new one or build a new one without replacing the body. Good news in this area for XJ's is one with a decent body is readily available and dirt cheap.
Personally i think that a 4BT is a lot for an XJ. If you want a unibody Jeep, a better choice, in my eyes, would be a ZJ. The XJ's were the first AMC vehicles to be computer designed, also the first unibodies. There were a lot of advancements in design from when the first XJ's were designed to when the ZJ was designed, the ZJ unibodies are a lot heavier and stronger than XJ's. Personally I'd still rather have a frame.
Can it be done? well that's already been answered. Will it hold up in the long run? if done right and not abused, probably.
I know you already have the XJ so it's an easy platform to choose. It also depends on what you want it for. If it's a weekend warrior/DD then it will probably do the trick. If you think you want to tow things somewhat heavy, you might want to consider something stronger and with a longer wheelbase. This is how I ended up in a Durango. A solid front axle is pretty much a must in order to have 4wd, but That's not a terribly hard/expsnsive operation either.
When it comes to money, it's not a cheap swap. It's definately cheaper to stroke the 4.0 that's in there now than to put a 4BT in. Unless a 4bt falls into your lap, then it still might not be cheaper. Will you save it all in fuel prices? maybe, but it will probably take a while. reports on mileage of a stroked 4.0 are pretty decent from what i've seen.
Sorry if this is a lot of rambling, hopefully something decent came out of it all.