If it is a grand cherokee, I would avoid the rear axle all together (assuming it is a D44)
I would start by deciding on wheather or not you want to build the jeep, and if the drive train you have is what you want to move forward with, then plan around that. Once you know what your gonna run, get some axles under the jeep. I am not sure what the out put is on that 231, but I will assume it is the same as a jeep 231 so... I would search for a FSJ wagoneer axle set with drivers diff, or a first gen bronco with a 44/9" combo (later being exactly what I would source). Then get your drive train stabbed in there and build away.