It’s just the weight of the locker hardware itself.

The weight of the locker hardware is negligible. However, the torque that can be applied to one axle shaft with a locker, is significant.

I've never seen a definitive comment from Toyota regarding why the payload is reduced so much with the locker, but limiting the potential load on a single axle shaft by reducing payload, makes sense.

What you said sounds right. It must be so you don’t destroy the rear axle shaft when locked under worst case conditions. I’d imagine if you don’t engage the locker you could increase your payload to the 1165 lbs spec.
Looking around the forum it seems no one has found the official answer but this thread and the @Jetboy posts cover your assessment, and it makes sense.
Vehicle weight capacity with Diff Lock

Does the lower payload of the TRD ORP have anything to do with KDSS?
I have KDSS and the rear locker and my stick says 880 lbs.
Will KDSS be damaged/affected if you overload the vehicle or tow?

Old thread but my 2015 SR5 4x4 with third row has the 12 lbs yellow reduction sticker and the 1165 payload rating.

I would have thought an additional row would have reduced the rating significantly. My 3rd row has since been removed and converted into fridge / proper storage space.

This really got me wondering if I was approaching my payload / gvwr with mods and newly added RTT so I took it to the Cat Scales. With me not in the vehicle, half a tank of gas and all my permanent items in or on the rig:

Front Axel weighed 1340, rated for 3000
Rear Axel weighed 2660, rated for 3450
Gross weight 4000, GVWR of 6300, 2300lbs remaining

My guess it to toss out what the scales are reading as I am 400 lbs lighter than the lightest factory curbweight. I have 200lbs on my roof, 150lbs of armor, at least 20lbs heavier per tire x5 and 150lbs of items in the trunk. With my 3rd row removed Im guessing I am at 500 lbs over the curb weight...

Anyway according to the payload I'm probably right at capacity when I get in the car with the gf and camping gear. Will try to get another weight elsewhere.

Edit 4/22/24: I was able to measure the front two tires and then the rear axle on a calibrated freight scale.

Drivers Tire 1513, Passenger 1369 for a total of 2882
Rear Axel 2633
Gross 5515, GVWR 6300, 785 Remaining

Im guessing the "weighmaster" at the cat scales fudged up my front axel numbers or they are in serious need of calibration.

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It seems to me that there is WAY to much worry and consternation about the load capacity stickers on the door.

The real crux of the matter is the vehicles Gross Vehicle Weight Rating.

I will use my 19 Pro and my Airstream basecamp 20 as an example.

When we left to go to Big Bend NP in January of this year, the CAT scale, 1st weight showed the following:

Front Axle: 2920 lbs 80lbs under max axle capacity
Rear Axle: 3640 lbs 190lbs OVER Max axle capacity
Trailer: 3800 lbs 500lbs under max load capacity
TOTAL: 10360 lbs fuel, 300lbs of people, and assorted stuff for a week in
the desert etc. etc.

NOTE: we DID take the time to move some stuff into the trailer once we saw the weight. We were at 3430 on the rear after we moved some stuff around.
NOTE 2: the trailer tongue weight on the weight distributing hitch is 490 lbs.
That is part of the truck's weight once the trailer is hooked up.

Final weight:
Front axle: 2930 lbs 70lbs under max axle load
Rear Axle: 3430 lbs 20lbs under max axle load
Trailer: 4000 lbs 300lbs under max axle load
TOTAL: 10360 lbs

Toyota Owners manual says the GVWR on the 4runner is 6300 lbs...3000 front axle, 3450 rear axle.
Airstream says the GVWR on the trailer is 4300 lbs
TOTAL Gross COMBINATION Weight Rating 10600 lbs.

So was anything over gross...NO!

Close, absolutely! That said the truck has heavier springs front and rear as well as a dual core radiator, transmission cooler, larger rear LCA's, 4.88 gears, a frame mounted WD hitch, and E load rated tires.
The 4runner tows nicely at 60mph in 4th gear and gets between 10-11mpg doing it. I set it up to tow, and I am still within Toyota's GCWR numbers.

We all have WAY more load capacity available in our 4Runners that we can use most days. Don't leave your wife at home because you are worried about the extra rack bars or your axle locker....

AMLOR

That's nothing short of a miracle that you are right at or under your ratings with your mods and towing. Does it include passengers?

I think the takeaway is GVWR is your engineered limit but you will need scales to verify. Payload works for quick and easy numbers but if you exceed it you may still have leftover capacity according to GVWR / axle ratings. I would assume you would be legally held to the GVWR / axles.

Toyota de-rating payload and blanketing all models into the 1165 vs 835 group is lazy and I wont try to guess reasons. My third row, kdss and other options seem to have little to no affect on payload but for some reason the port installed options and a locker get reduced payload? Very inconsistent and no wonder there are multiple threads on the topic.