You can use it in any gear but I find it pointless - and I have read that it contributes wear and tear.

I generally use it just as an overdrive, bunging it in when I'm in 4th and hit 40.
 
wouldnt use it off road in 4 wheel drive low ratio, as you want a low gear and not to stress it too much
 
Its intended as a real overdrive. 5th gear in 2wd.
I remember reading somewhere that using it in any low gear might strip the cogs.
 
the overdrive is before the transfer box, so the load will be the same if it is in low range as in high, in fact it could be less
 

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