Future mods for my 1990 Geo Tracker:
I want lower t-case gears.  So far, looks like Trail Tough is priced right, plus they offer a set-up-in-the-case option.  I just need a donor case 91+ since they won't fit my '90.  So I got that squared away.  I just need a 91-early 95 case so I have the proper output shaft to keep my driveshaft the same.  Otherwise, late95-98 has a fine-spline that is different (from what I understand).  So I've got that figured out. I'll install a twin stick too.  I know Stan and Rock4x both make excellent kits.

NEXT: I want to run front and rear ARBs. 
If I am running the stock 10-bolt rear axle, what ARB will I need? RD74?  I will also upgrade to the heavy-duty rear axles at that time.  This axle appears to be less complicated to get an ARB in there.
Front end: I want to upgrade to the Anvil housing.  I have the Calmini 3" suspension (and their front skidplate).  I'm assuming I can keep the front skid with the Anvil since it clears my current aluminum housing. 
Correct me if I'm wrong: it uses a third member from a newer Vitara/GV?  Then, what types of axles would I need to buy (both driver's side).  THEN, what ARB is used for this application? 
Finally, once these mods are done, I assume I will not have to change any diff gears to work with the ARBs or the new Anvil housing?  I definitely want to keep my 5.12s and I don't think it'll be a problem, but wanting to be sure of it before I decide to do any of this.  I want the 5.12s because they drive great on street/highway.  any lower, and I'd lose my 5th gear cruising RPMs.
I've tried researching this stuff on sites but it's a lot of stuff to piece together and I've had trouble getting info.  If someone who has done all this can give me some advice, that'd be greatly appreciated. 
Thanks
Daniel

Daniel,
I'm not sure on the ARB questions (always ran auto lockers) but:

Correct me if I'm wrong: it uses a third member from a newer Vitara/GV? You do not need to use a newer model vitara 3rd, the stocker will work....BUT.... I would recommend it the aluminum third will be your weak spot.  Then, what types of axles would I need to buy (both driver's side). The Anvil will come with the inner stub axle you need and the outter passenger side will switch to a stock drivers side.. so you will run a drivers side CV axle with the 3 bolt flange on both sides.

There is a good and easy upgrade to Toyota CV axles if you want to make it bullet proof (Stan makes them so shoot him a PM to 37Kicker if you are interested.

I believe you are correct about the skid. It should fit with the anvil. At least fit with a little modification.

HTH,
Zig

PS.. More Anvil info here; http://www.zukiworld.com/month_110103/feature_zigsbeef.htm

Hmm...  Seems when I put ARB's into the Gnewt, they were RD88 and RD89's.  That's in a 99 GV Steel carrier, in the Anvil up front.  Make sure you get a carrier from a GV with MANUAL transmission, for the steel unit.

I also have the Calmini heavy duty rear axle shafts, and the adapters to run Toyota IFS axles up front.

I have the 5.83 ring & pinions, and the Calmini 4.24:1 transfercase gear set.

I cut my fenders and put on TrailTough fender flares, to clear my BFG 33 x 10.50 MTR's.

Thanks Zig, I will consider that, also might buy my twin stick from Stan too in that case.
And Bob thanks too.  I guess Hawk has the right carrier already set up for install in the Anvil?
I definitely still have some more research to do!