I'm not sure what bit of the differential assembly you mean, but nonetheless, only the rear is interchangeable.
I'm not having a go at you at all Luke - everyone is guilty of doing this, including myself, but I'll just put this in here for the sake of clarity.
This is a diff centre
This is a differential carrier
And for the sake of clarity, I'm going to use the american terminology for the assmebly, which is "third member." This includes the ring and pinion gears (not "diff" gears, they are contained inside the centre)
The sierra rear third member will bolt into the Jimny axle housing, but it will have the wrong diff ratio. (3.7 or 3.9, not 4.1)
The sierra carrier will accept the jimny gears and centre,
A Jimny carrier and gears will accept a sierra centre.
I do not believe ANY parts of a sierra differential assembly will fit a Jimny front.
Sorry if this seems confusing, but all these parts have different names and everyone seems to use them without much thought to what specific bit they are referring to. It leads to confusion and misinformation.
Steve.