At this point you can carefully drive the Jeep forward and have a friend look to see if the shafts are turning. If you hear crunchy noises coming from the differential then there is your problem. If not you can try to see if the axleshafts are being driven by the wheels or the T-case. Do this by unlocking hubs (if you’ve got ’em) and also by shifting in and out of 2WD. Now you should be able to figure out which side is broken and/or if one of the inner shafts is broken. If the axle U-joint turns, but the wheel won’t then you either have a broken locking hub (if you’ve got them) or a broken outer (stub) shaft.