The rear axle on a J-Truck should be roughly 66.5" WMS as I recall. The rear axle is actually wider than the front. Nothing narrower will work unless either link the rear axle, or move the springs under the frame.
As I recall a JL Rubicon rear axle is 66.5" WMS. So, they might fit. But its possible the calipers will hit the springs. As the J-Truck rear axle has drums that are narrower than the disc/e-brake setup on a JL axle.
The front Could be an issue if you are wanting to keep your truck the same height. Up front, the axle is a spring under, and I think the pumpkin is going to be in the way of where the spring wants to live. You might have to outboard the springs, which is something people do if using super duty axles.
But as noted above, the M23 (FSJ version of the M20) is stronger than D44's of the era. Not as strong as JL Rubicon rear D44, but its not a giant difference.
tgreese wrote: ↑Fri Feb 02, 2024 8:06 am
A J-truck Corporate model 20 is not the same as the CJ model 20. Not a bad axle. IMO what you are suggesting is not an upgrade; instead a sideways move.
The JL uses AdvanTEK "D44" axles. They are a D44 in name only, and not comparable to the D44's that came on FSJs (no parts interchange). The internals are quite a bit stronger than older D44s.