I'm not sure if the 231 will bolt right in or not, you may have to do some modification to your driveshaft to make it fit. What year is your ZJ?
The cool thing about the 242 is that it has 3 4wd options while the 231 has 2 (basic 4hi, 4lo). The 242 has a 4wd full-time, which is more like an awd, where both driveshafts can spin at different speeds and wont bind up on you like normal 4x4. It also has part-time 4wd which acts like regular 4x4. Having the option of full-time 4wd is ideal for wet roads, etc. Do some searches on both cases and you'll find what better will suit you.