We don't have the answer because I doubt anyone has ever done it.
You can update the Internet when you have done it with the outcome.
Actually my question was if it was a crazy idea, not if anyone had done it.
I received a lot of abuse without any actual substance, and some abuse based on data but the data had nothing to do with the conclusion that caused the abuse.
Essentially people who did not know the answer decided that the idea was stupid and attacked me for the idea.
But given that I received a lot of criticism that was not based in any actual factual truth I am confident that there is not some obvious underlying problem that I missed.
I also received the wonderful information that all of the part numbers are the same and the subframe is the same so the likelihood of the different diff fitting is very high.
On that basis it is not a crazy idea to buy one diff from eBay or a parts seller and see if it fits, and if it does then buy a second diff and try out my idea.
I also received excellent information from RRPhil on another form that my second gear would be close in ratio to the first gear that I would get with the modification.
This means that I can drive my car in second gear and see how it pulls away from the lights and see if that bothers me or not.
So my question was answered.
I never asked if anyone had done this before, I never asked if it was a guaranteed outcome, I just asked if it was a crazy idea.
If someone had told me that there is no way that the diffs would fit in my car then I would know that it was a crazy idea.
If someone had said that the engine management system would have issues with the different ratio then I would know that it was a crazy idea.
If someone had said something that I had not thought of that would make the idea not work then I would know that it was a crazy idea.
Instead what I received was people telling me that it was a crazy idea without any reason.
A lot of people fixated on fuel saving which I made clear many times was not my concern.
In summary people pointed out that the car will accelerate slower away from the lights which I already knew, and don't care about.
Some people said it might not shift into fifth gear but I explained that my car has a manually controllable automatic gearbox which can be forced into fifth gear.
Something that anyone commenting here should already be aware of, but none of them seemed to know anything about the gearbox in my car despite coming here to tell me how my gearbox would behave.
Some people said that on a steep hill I would have trouble pulling away in first gear, but I explained that my car has a low range gear and I could use that in those rare situations.
Again, something that anyone commenting here should already be aware of, but none of them seemed to know anything about the gearbox in my car despite coming here to tell me how my gearbox would behave (I am including the transfer case as part of the gearbox, where the low range gear resides).
I also explained that I am not towing and I will not have a lot of weight in the car so I do not need the maximum acceleration of a car that was designed to tow heavy weights.
Despite all of this, people told me that Range Rover knows best and there is no way that anyone is allowed to modify cars because nobody modifies cars anymore.
That is not a joke, someone actually made that comment and argued that point.
I was told to put an LPG system in it, change the tires, change the exhaust, buy a different car, take a ferry, etc etc etc.
A lot of people spoke about putting load on the engine when I never said that load on the engine was an issue for me.
I spoke only about wanting to drive at a lower RPM at a constant rate for a long period of time on a long trip to Portugal and back.
Basically people who thought it was a dumb idea because they didn't like it decided to attack me without any substance or understanding of my needs.
Eventually I responded to those insults, those ridiculous statements, and the patronizing comments because it was just such a ridiculous situation.
But I did get my answer and I did get to have a lot of laughs at the idiots here.
But the best bit was the people who said I would never find the answer to my question the next two posts after I had just explained that I found the answer to my question.
Anyway I wasn't going to comment more but this has been so funny that I just couldn't resist having another go.
Please continue to try to tell me that I didn't find the answer to my question that I have already had answered, because you didn't think of the answer, perhaps?