Hi,
Stage one diffs are 3.54:1 (rover front, salisbury rear) which is the same ratio as defenders.
There weren't many Stage ones made so parts are fairly rare and stupid people like me who own them want spares, so they can be bloody expensive.
Saying that I get the feeling the rover diff might be same as an early RRC 10 spline one... I can't remember...
As far as changing diffs it should be fairly simple although if you've got a salibury rear end, I think the diffs can be annoying to change, as far as I'm aware you need to spread the axle apart to get them in or out.
One thing that other people might be able to advise on is how it'll change the performace of your truck.
Don't quote me on this but, I think, while it'll work your engine less for things like motorway driving, the ability to drive offroad, with a reasonable amount of weight in the thing or while towing will be badly effected.
Found this with a quick google search, might be worth a read.
(
http://forum.landrovernet.com/showthread.php/194947-Diff-swap-options-for-Series-3)
