I guess size wise the 231 is smaller than the 242. BUT:

The NP231 Transfer case (Command-Trac) is a true 2 speed case with 3 settings: 2 Hi (Pavement use) 4 Hi (Off-road Use) and 4 Lo (Heavy/slow offroad use.)

The NP242 Transfer case (Selec-Trac) is a 2 Speed case with 4 settings: 2 Hi (Pavement Use) 4 Hi (Off-road use) 4 Full-Time (Essentially an AWD setting for pavement/Off-road) and 4 Lo (Heavy/Slow Off-Road Use.)

The NP242 is larger because, and only because, it has a viscous gear thing that allows the rear and front driveshafts to spin at different speeds (Slip) to keep the case from binding up in 4 Full Time.

The NP231 is considered by many to be the stronger case.

 

The better news is that you never tried to use Full-Time

 

Kool...that sounds good...thank u for all your input

 

i have a 95 with no abs, and an amc242 from 87 with an np231 bezel... i'm the freak of nature

 

My 90 has ABS and a 231.

 

take a look, maybe you've got the larger u-joints?
i've never heard of an xj with vac disco and abs.