There are some early Sidekick rims that won't fit the 2nd gen trucks either, even with the factory 4 1/4" back spacing.  I had a set of alloy rims from a '95 kick.  I mounted a rim to check for fit... it appeared good so I mounted my tires on them and drove them.  After about 20K miles I needed to install some new brake pads on the front end.  One I installed the new pads, the calipers were fully expanded and I suddenly had caliper rub on the inside of the rims.  I could have just "ground some caliper" for the clearance, but I elected not to.  There was no driving on these rims without modifying something.  They wouldn't roll/spin because of the caliper contact.

My point being, even if the back space is similar, it is the shape and mounting surface of the inside of the rim that can cause grief and fitment issues.  Just be aware of this.

Thanks so much for reposting the info on this thread captwoody!
I got linked here to the old thread that was lost and was like, nooooo, WTF?!
I signed up just to say thanks when I found this because I've been hoping to eventually put an ARB air locking rear diff in my 2003 ZR2 Tracker and have read that there is no drop-in solution unless I want to swap a 1st gen third member in. This info sure is helpful, hearing from someone who has actually done it gives me more confidence to carry on with that plan. Machining the ring gear doesn't seem too bad to me unless there is some advantage to swapping out the whole rear end instead. Do you have to modify the ARB parts in any way or just mill out the ID of the ring gear? I have to admit I'm not an expert when it comes to this stuff but I do have a firm grasp on the principles, it's the nitty gritty details and specifics of my rig that get me worried. I have sure learned a lot in the last year or so since I got serious about it. For some reason I really don't see many people doing much with these trucks, although I can't understand why. I'd like to help change that a bit by showing people you can wheel these things without having to completely re-invent them. I've been super happy with my ZR2 the way it came stock and all I have really done so far to make it more off road capable is putting in an OME suspension with the 989 springs (that add 1" of lift) and nitro charger shocks & struts that have a longer stroke and add a bit of articulation to the front end. I have some 1" coil spacers and put on 235/75R15 29" AT's on 1" wheel spacers and cut a bit of the plastic trim out of the way where it liked to rub. It wheels amazingly well considering it only has about 2.5" total increase in clearance. The only mechanical trouble with it offroad so far has been popping front swaybar links due to the increased front end flex (which went away when I switched to Moog adjustable links). Performance wise, I'm really happy with the way it drives on and off road with the stock 4.88 gearing on the 29" tires. Aside from minor nuances with any modification, the truck is loving it.  Having a selectable rear locker that doesn't eff with normal driving is about the only thing I would really want to add now, aside from a custom front bumper and winch setup that I have been putting off mostly for reasons of still being a bit too broke to afford it after all the money I spent on the suspension. I blew over 2 grand on it but I don't feel like a wasted a dollar. I looked at cheaper suspension kits like Calmini and for the extra money, so far the OME ride and reliability really has been worth every single penny, it really does live up to the hype when you notice how much better your ride handles in any conditions. I've done everything from washboard drifting to driving rally speed on trails and crawling washouts, and the suspension has taken everything I could throw at it with ease and it still rides so nice on the highway. A rear locker and a bit more lift (when it's not my DD anymore) would make this thing a formidable force off road as it can squeeze in to the tight places that would tear a strip out of a larger truck and the short wheel base and tight turning radius make it ideal for the types of trails and obstacles frequently encountered in my area (Pacific Northwest, Vancouver Island). I plan to keep this info handy and hope someone is still around who knows what they're doing once I decide (can afford) to go for a locker... CHEERS!