the cj axles were in the range from 50-56 inches wide i believe, depending on wether or not you have a narrow or wide track axle. the ford 8.8 is in the range of 59 inches wide (58.75). your rear axle is 55 inches wide according to some quick research. If you could find a dana 44 from a 86 cj7 you would be set, they are widetrack and were used in place of the amc 20. Like mentioned above you could use a scout rear axle, which is 58 inches wide. your front end is 55.8 inches wide so in order to run the scout rear axle and still maintain a balanced look and keep your steering in order, you would need to run 1.5" wheel spacers (scout axles are relatively abundant if you can find a junkyard with a scout or two, i know my local ones always have one or two scouts). or if you used an 8.8 you would have to run 2" spacers up front
The AMC 20 one piece axle swap is not a hard swap at all but, you might have to pay someone to do gears in the axles, so dont forget if you do one, you have to do the other. IMHO the best route to get is to just rebuild the 20 with one piece shafts and regear it with a good LS or air
locker. you can do the one piece axles yourself but gear installation is a tricky thing. it requires lots of attention, lots of patience, and plenty of time especially if its your first time. it is definately not something taht is recommended for the garage mechanic to do with a limited amount of tools.
in conclusion, my opinion is->
look for a 44 out of an 86 cj and jump on it if you find it. check ebay night and day, they dont go for too much usually.
otherwise, rebuild the amc 20 yourself with new one piece shafts (it will come with new bearings and seals in the kit), buy a limited slip or selectable
locker and have someone regear your axles for the bigger tires. go to 4.10s and youll be plenty happy.
edit, theres one on ebay right now if you live in or near illinois, its in good cond and local pick up