I believe but not 100% that you'll need one of these kits to fit Vitara 5.12:1 gears into a Sierra 4 pinion carrier.
You can use a WT or NT rear diff or a NT front. They are all 4 pinion.
You can only use an NT front if your lockright came with sidegears.
[Quote="lowrangeoffroad"]
Carrier Pin Retaining Cans
Precision CNC machined parts make this kit the best on the market. This retainer sleeve and bearing adjuster are needed for installing Suzuki Sidekick OEM gears into a Samurai carrier. You will also need the retainer sleeve when installing aftermarket 5.38 Gears. The reason for this is the cross pin in the carrier is held in by the ring gear. With the thinner Sidekick gears and aftermarket 5.38 gears, this is no longer the case. No welding or machining is required for the installation of this kit.

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http://www.lowrangeoffroad.com/samaxles.htm
Jas.