NP231, like the NP242 has a couple issues. #1, it comes in 21 or 23 spline input, #2 it has a deeper input than the Eagle or FSJ tcases (15/16") so if you just toss it on the Eagle trans, you'll have only about 1/2 the spline making contact. #3 is the slip yoke output (fixable with a slip yoke eliminator). You still need new driveshafts to get the right length AFAIK.
Jeep NP208 is another option, similar to NP231, but you only need a longer rear driveshaft (it's about 5" shorter overall than an Eagle's tcase but the front yoke is the same position)