231 is known as 'command trac' , its been in wranglers for over 20 yrs, its a 'part time' transfer case so its only to be engaged in 4high/4low when wheel slippage will happen ( off road or snow/ice roads ) - it has lots of aftermarket support and is pretty dependable ... it also appears in some other vehicles ...
242 has been around for a long time as well and is known as 'select trac', I had one in my '95 cherokee and my liberty has it, its a 'full time' transfer case - it has the 4high/4low like the 231 that requires wheel slippage but it also has an extra 'full time' selection available on the gear shift , in this mode you can drive on all road surfaces (ice/snow/nice and dry) and not have to worry about it other than gas mileage will suffer - it handles the slippage it requires internally/automatically ... it doesnt have as much aftermarket support - as far as I know it too is pretty dependable ...
edit: the 242 never came in a wrangler stock ...