GQ rear diff ratio's

you are on the right track... it would be 8.2 : 4...
And you would probably need 4 times the hp to reach that speed...___________________________________
Simpson trip 05/04 then turn left at Birdsville to Darwin via Lawn Hill etc
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Look on your compliance plate it will say HG39 HG41 HG43.. thats your diff ratio.

If your standard diff is 4.1:1 ie four turns of the drive shaft to one on the axle you cant replace it with a 2.2:1 as it will not match your other diff. Both diffs have to be exactly the same ratio or you will get transmission wind up in 4WD that is the front diff fighting the rear,even different tyre sizes can cause this to some degree.

As Nav80 mentioned you would also need to change front diff ratio as well.
Also I would not go lower than the 4.1 ratio.
Preference would be the 4.3 or better still the 4.6 if you are running large tyres.

With 2.somethings I doubt you would ever get it to move, you would need to slip the clutch something terrible to get it to take off.

So you have a factory GQ Locker correct??

They came out in GQ's with 4.1s standard. They came out of *SOME* Ti Model GQ's, and werent that many of them. They are Vacuum operated, there should be a solinoid on the diff housing, a small black square box thing...

Failing that there would have to be a part # on them, or if there isnt take to Missan and ask them.

Fordpatrol.
If the diff came out of a TI it is 3.9 ratio and you can change the crown wheel and pinion over as long as it is the same housing, some housings have 12 bolts holding them into the housing and some have 9 bolts, as long as you new and old are the same you are ok.

Fordpatrol
I'm assuming that when you turned the driveshaft on the standard diff in the car that both wheels rotated due to it being an LSD. When you turned the yoke on the locking diff were both wheels turning? From your results I 'm guessing only one wheel was turning. This is because it is not an LSD and behaves like an open diff when not locked. The nature of an open diff is allow to different speeds between the axles when cornering up to a ratio of 2:1 to the driveshaft. So if one wheel is completely stopped then all the torque goes to the opposing wheel which will be turning twice as fast as when equal torque is applied to both axles. This being true when you turned the yoke on the locking diff it should only take half the number of turns to rotate the wheel once if you have the same diff ratio (and isn't locked). That is 2.055 turns for a 4.11 diff. Just for interest sake jack one front wheel off the ground, lock that hub and turn the front prop shaft and see how many turns it takes to turn the wheel once. I'll think you will find it takes 2.055.
Cheers