I bought a WHOLE NP231 case from a junkyard for 150... pick and pull, I had the NP242 which is a decent setup but I wanted the 231 for ground clearance since the 242 sits about 2.865 inches lower (round about figure) the 231 for some reason holds up better not sure why or how but Ive NEVER had an issue with a 231 and had MANY problems with a 242 (seals oil pump output shaft speedo gear chain... chain chain, and a shift lever inside the case kept popping out of 2wd into neutral)
my .02cents saying all that to say... might be cheaper to get a 231 from a JY.. and if that IS the 231 you have ... just do what muddy said...