Take it out of 4x4. Now reach down and lock the diff lock. Will it pull? If so, you probably have a stripped hub. Have someone watch the rear axles while you try to drive in 4x2 with the diff lock unlocked. If a hub is stripped, they'll see that axle turn but the machine will not move.
New hubs are sold in pairs now as I remember. The hubs are softer than the axles so that you can replace a hub rather than axle. Axle replacement isn't much fun.
