I have an XL7 and my question to you would be, are you sure you want to go that route? It will be a bit more work, but I think a Ford 9-inch would be a far superior swap for the following reasons:
1) The XL7 housing is going to be the same width as your Sidekick. The overall vehicle is the same width - I have a Sidekick bush bumper on my XL7- it's just that the framerails are farther apart on the 2nd gen Zuks. So the axle is not wider, no advantage there. A 9-inch would definitely be wider.
2) The XL7 differential and shafts are indeed larger than anything else Suzuki ever made, BUT...this is what makes them impossible to get a locking diff for! It really sucks not having any options other than to "weld'er up". Compare this to the Ford 9-inch which is arguably THE most popular builder's axle, both for drag racing and off roading. More gear ratios and lockers than you know what to do with, and there are lots of people who know them inside and out who can help you figure out what's best for your rig.
3) The XL7 pinion yoke uses a different bolt pattern than a Sidekick or Vitara. It's a 6 hole pattern, of which 3 are used. The XL7 driveshaft uses a CV joint similar to the front half-shafts at the pinion and at the steady bearing. That's right, it uses a steady bearing,therefore it's a 2-piece driveshaft, so it's not like you can just swap the driveshafts with your Sidekick. Here is a bit more info on that unique driveshaft:
http://www.suzuki-forums.com/1g-2001-2006-xl-7/49901-rear-driveshaft-vibrations.htmlMaybe it would be possible to fab up an adapter so you can use a regular 4-bolt driveshaft on the XL7 6-bolt flange. Or else just get the end of your driveshaft cut off and have the XL7 end welded on at a shaft shop.
4) Don't forget that the XL7 uses a 4-link with a track bar to hold the axle in position. Your Sidekick uses a 3-link, the 3rd link being a V-shape with a ball joint on the top of the diff that holds the axle side to side. So you are going to have to do some fairly significant measuring and fabrication, especially since the distance between the framerails of the 2 vehicles are different. It's not as easy as you'd first think. Not impossible, but going back to my original point- it wouldn't really be any MORE work to put in a 9-inch which would be alot better, especially if you're going off road.
5) The factory rear diff is fine if all you're planning on running is 31's. Most drivetrain issues people are having revolve around front ends, not rear ends.
6) If you want better braking on your Sidekick axle, you could always install a rear disc brake conversion. It would probably be possible on an XL7 axle as well, since it still uses the 4 bolts on the end of the axle to hold the wheel bearing and axleshaft in, but nobody makes a kit for that so you'd have to fab it yourself. The only thing with that is, most companies that sell the kits don't address the parking brake. Some people run line locks which I don't think are legal. Another method is to install a parking brake on the rear driveshaft. I think some old Sammy's ran that, and 40-series Toyota land cruisers.
If you haven't given up on the idea yet, and you do need some measurements of the diff or housing, let me know and I'll see what I can do.