9 inch axles don't match the Jeep bolt pattern. That's why they aren't as common for swapping. You need to put extra effort (money) into either changing the pattern or swapping in a front axle too.
Oh, and I broke something in my 88's 9 inch last offroading adventure. Don't know what yet, but the pinion is just a bit looser than before. :hmm: Can't touch it though, as it's in line behind my '90 for a turn in the garage.
Nothing is invincible, but the 8.8 is right on the 9 inch's heels for toughness.