Plenty of options exist for lockers. LSD’s basically don’t exist.
Correct, with a front locker you’ll want a steel diff.

I agree, LSDs are rare and can be eye wateringly expensive. Not that air lockers are cheap, but they are more effective.

I'd just put an air locker in the rear diff and not worry about the alloy front diff. With mild wheeling and mild tyre sizes it's very unlikely you'l hurt it. With the rear locker you wont be getting cross axled and shock loading things or left foot braking to get through, the front diff will have an easier life.

Installing an air locker is very simple, it's hardly any more complicated than fitting any other kind of diff centre. There's plenty of guides on various websites. ARBs kit comes with everything you need except the compressor which of course they also sell separately.

The rear diff is plenty strong enough to be locked for your use, the ring gear is the same size as a Hilux one.

I agree, LSDs are rare and can be eye wateringly expensive. Not that air lockers are cheap, but they are more effective.

I'd just put an air locker in the rear diff and not worry about the alloy front diff. With mild wheeling and mild tyre sizes it's very unlikely you'l hurt it. With the rear locker you wont be getting cross axled and shock loading things or left foot braking to get through, the front diff will have an easier life.

Installing an air locker is very simple, it's hardly any more complicated than fitting any other kind of diff centre. There's plenty of guides on various websites. ARBs kit comes with everything you need except the compressor which of course they also sell separately.

The rear diff is plenty strong enough to be locked for your use, the ring gear is the same size as a Hilux one.

Agree with 225/75R16 being the most useful size to run.
SQ420 will be alloy front diff - you don't say if it is a manual or auto, but I would rather have a locker in the rear diff any day over the front.
'Mazing on the different thoughts on issues like this.
I would say 2.0litre (+ auto?) driven like you own it, will be unlikely to bust the front, even with larger, gripper tyres. Having said that, peoples do break these diffs when in earlier, lighter, less powerful 1.6litre Vitara's, so it is possible.
Yes, you can swap in a steel housing & centre from a manual V6, but has 4.3:1 gears, which isn't what you will have = set up costs.
Some of the last auto V6's had a steel front diff & run 4.875:1 gears, which is what you should have as a manual. If you have an auto, should be 5.125:1.
Yes, the front has ordinary travel (along with pretty much all IFS 4WD's), but longer shocks & springs in the rear will give decent travel, & give you decent drive with a locker.
An air locker is $1000, fitting it (by a workshop) should be $350-400, Compressor $300, wiring & plumbing $300 - you are realistically looking at a $2k excersize.
Try a set of M/T's for 1/2 that amount & see if you really need it.
As a touring / mild 4WD (not rockhopping, as you say) there will be few times where a locker is worth having, but if you have the money, go for it.
The P/S/F driveshaft is not too hard to change, but the D/S/F is a PITA at the best of times & there is not a good way of running a P/S in the D/S with a stubby mod, like can be done in a Vitara.

The lightest car is J20manual (I suppose we are talking LWB wagons here?) at 1335kg curb, J20auto is 1375kg & H25 man & auto are both listed at 1405kg which is a bit strange.
The V6 has noticeably more go than the 4cyl, but drinks more fuel. both have 66litre tanks.

OME from ARB is as good a suspension kit as any, although Dobinsons & Pedders have options as well.

Have fun, Rgs, Michael

Why do you want an LSD?

The 108/118mm refers to the ID (internal diameter) of the ring gear - all of the Vitaras, and Grand Vitaras up to 2005 should have a 118mm ID ring gear in the rear axle - you can pull your third & measure to confirm.

Better than an open diff. Good for gravel and slippery surfaces.

So should fit. When you say remove third, do you mean drive shaft? I wouldn't know what to measure. I have an SQ420 2004

Maybe. An effective LSD will improve acceleration out of a corner no doubt, but will substantially increase oversteer, especially if you're cornering close to the limit of adhesion and then accelerate. I guess it depends on your focus, but if the LSD won't drive the car with a wheel in the air, all it's really doing is tuning the driving experience, it's not making much difference to capability.

As sideways points out, they're great for sand though for a completely different reason.

As an aside, I'm finding some "interesting" interactions between stability/traction control and the LSD in my FG ute, especially when the LSD is making the car slide and then the ESC tries to correct the slide.