J20 axles should be 6 lug especially if they are both front/rear D44's. These will be the same as Wagoneer axles. J20 is a 3/4 ton Jeep pickup. Some come with D60 semi-float rear axle or a full floater rear axle. As such, the axles would be 8 lug. I would guess that if both front/rear axles are D44, that isn't stock for the J20. J10 on the other hand, would be a D44 front/rear. But be careful, J10s are known to have quadratrac tcases which means the output to the rear axle is offset to the right. If this is the case, you could still run them, but you'd probably have to redo your exhaust to the driver's side to clear the angled rear ds, if you still plan to use the D300. Not much else out there for offset tcases, unless you find a Dana18 for an ealy Jeep. These are offset tcases, dual sticked, have low gears available, and are real short in overall length.
Good luck and take pics of your findings. Junk to treasures always look nice.