J20 should be a T18. Transfer case, if aluminum (np208), is nothing to brag about. Could be a Dana 20 if the truck is a 70's model, still nothing to brag about. The late J20 T18's are quite odd, as they are a Ford in the back(31spl, ~7" extension housing) and Jeep up front (odd mounting pattern, long input) and they also have no pto cover, it's cast into the case (Yes, I have confirmed this personally). Earlier 80's FSJ T18's are the same spline as the Dana 300, but came with a NP208, which isn't anything great. Either way, if it's an 80's model, with the long rear extension housing, it's almost better to start with a Ford T18 for a swap into a CJ.
1977 Cj5, 258, T-18, 2.46:1 Dana 20, 2.5" BDS lift, OBA, 38.5's. Chevy 60 frt, 14b Rear, Full hydro, 102"wb, full DOM cage.