Howdy,
I recently installed some larger AT tires (235/70/R16) on my 1999 GV, but in doing so I did notice some power loss in 5th gear.  From my calculations my gearing is now around 7% taller, but a 4.625 ring & pinion will bring it back to stock gearing.
-My GV is a manual transmission model with the 2.5 liter V6, and hence has the 4.30 ring & pinion and the steel front differential & housing.
-This vehicle is not lifted, and I currently do not foresee lifting it.  It will primarily be used on the road and on mild to moderate trails, muddy dirt roads, logging trails etc.  Easy stuff.

From what I recall, the first Generation diffs will more or less bolt into the early 2nd Generation Vitaras.  I have several sets of Sidekick 3rd members in my parts stash, so the cost will largely be contained to my time. 
-I was thinking of first swapping over a 4.625 rear diff from a First Generation Sidekick (10 bolt without the abs ring/sensor).  The GV has the ABS sensors on the axle ends, so I believe that a 10-bolt Sidekick rear diff will bolt in to the GV rear axle housing without any issues, and the ABS will work as before.

-For the front diff I can either use an aluminum 4.625 diff with 26-spline sidegears (96-98), or I can swap the 4.625 gears over into the steel GV differential housing/carrier with a spacer and ring kit.  I am actually thinking of the former (26-spline aluminum diff) as it will be less time consuming.  I would also like to remove the vacuum assisted GV front diff, and go to a strictly mechanical system utilizing the Sidekick 2-pin carrier and manual locking hubs.  I realize that the aluminum diff will be considerably less strong than the steel diff, but again I do not plan to ever run a locker or do anything extreme offroad. 

Anyone have any comments or suggestions as to why this will or will not work?

What you're proposing, the replacement of the front diff center with one from an earlier model has been done by others - there is a write-up on the forum about it.  You will probably need to "fiddle" with the front axle shafts that fit into the diff center because the pneumatic freewheel mechanism (it does not use vacuum) is offset to one side.

Fordem is right (again).  If you use an earlier model carrier it will require the use of an inner axle shaft from the earlier model Track/Kick as well.  Because of the difference in the width between the 2nd gen air actuated diff and the 1st gen standard diff, there is about a 3/8th" (10mm) difference in length between the two inner axle shafts.

In the pic below, the 1st gen inner axle is on the left and the 2nd gen axle on the right.

Thanks guys for the info. I searched on this forum, but I didn't find any discussion on the topic. Maybe it was lost along when the great data dump hit a couple of years ago? (Back when 10 years of the forum was lost.)

Next related question. Can I simply bolt in the front aluminum differential assembly from the first Gen?  That is, the aluminum IFS axle housing?  That way I would not have to break anything down.

Should bolt right in.  You might have to swap the diff drop brackets.  The aluminum diff and axle housing should be just fine with doing what you plan to do with your truck.  I would fab a strap for up and over the nose of the diff on the cross member though.  Once you have the nose (pinion bearing side) of the diff secured, they are pretty sturdy.  No metal likes to flex too much, especially thin aluminum cases.  Make sure all your bolts (on the axle housing, brackets and diff mounts on the cross member) are snugged up correctly.  Some blue Locktite would be useful.

I ran my '91 4-door kick pretty hard at times and even pulled a couple cars out of snow banks using reverse.  It held up fine.  My '02 has the diff strap over the diff snout and it has withstood the abuse too... so far.

We lost MANY fine articles on "how too" from the data base crash a while back.  Hopefully we can rebuild/restore much of it over time, but some info is lost forever.