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As a stop-gap, find ANY junkyard axle out of the same vehicle and swap. They're not hard to replace as a unit.

EDIT: Any leaf-sprung Montero/Raider with a four speed automatic or a 5-speed manual after 1986 will work. Pre-87's (at least with the automatic 3-speed) came with a different differential ratio.

Something's either not fitting correctly, or there's a scratch/debris in there that may (just maybe) have been missed.

John B.
The edit part is what is important here. I highly recommend looking for one with the air locker and the disc brakes. If you've got big tires ( I have 35's) When you get the swap done, you will be amazed at how much better it stops.

89 SWB, V-6 5SPD, OME Shocks, 16.5X37's, 2"BL, SR axle, Gen2 brakes, Lincoln w/Superwinch