A technical question from experts.
I have this modified 1.3, efi, 5 fwd manual with 11:41 (3.7 diffs) in my stock jimny sierra, modified lwb which i am running on 235. 75 R 15 (overall diameter/ height 28.5").
Now when i drive this veh between 100/ 110 km/ hr, the rpm remains around 3000 which looks very low to me. And once the veh has 4 ppl with some stuff in it, on highways at a speed of 110km/ hr, in 5th gear the veh needs 4th gear to touch 120km/ hr.
Having said all this, here in pakistan, we have both the other diffs available, of the same model ie 11:43 (3.9) and 11:45 (4.1). Pls suggest if at all i have to change my diffs, whether to go for 3.9 or 4.1? No hardcore offroading required, just reasonable power in low gears and the veh should touch 120 with the same tyre size.

You might want to just look for a transfer case from an SJ-410.  That is going to get you a 10% reduction in the high and low range gear set and should be a lot easier then replacing the differential gear set. 

Yeah, what Kris said... Do the T-case swap instead of diff gears as you get low range AND high range reduction all in one package.

Good luck!

-Eric