I always thought that it was the best mod I ever did to my 4/4. I had Mike Duncan do it. He sourced a used set of cogs that were in great condition and used a rally transmission company in my local town to swap the diff. All in, my cost was £1100 and was money well spent. The back axle was quieter with the new diff!
I've driven a few of the later Mazda gearbox 4/4s and the gearing feels different so I don't know if the 4:1 is a worthwhile mod for those cars.
