costs, other thoughts
I had one third member converted from 3.70 to 4.11 and I supplied the parts (total rebuild kit and gears). It cost me $230 labor, which was higher than what Kurt at Cruiser Outfitters had quoted me ($175) but the shipping would haved killed that deal. Thus, if you need to have 2 of them set up, plus a locker install, its going to cost something. Hopefully, when you ask about two diffs, the price will come down some.
The other costs associated with the deal are gaskets and seals. Rear axle rebuild kit is $65.60 plus shipping (new wheel bearings, seals at Cruiser Outfitters). Install kit for each differential (bearings) will cost you $110-$120. That is not including pinion flanges at $37.50. Would you want to do rear brakes??, New gear oil (Mobil 1 synthetic 3 quarts=$21). Grease, brake cleaner.
I assume that you are going to remove and install the thirds yourself. If so, it would be smart to recruit an extra set of hands for the removal and replacement of the 3rds..they are heavy/awkward. Also, check the condition of the studs on the housing to make certain that the nuts can be hand threaded on BEFORE you put the new third back in. The threads on mine were damaged and I did not know it until the 3rd was in place and the gasket sealed up. No, there is not enough room to use a re-thread die with the 3rd in place.
You will need a slide hammer to help you get the wheel bearings out properly; try to borrow that item if you don't have.
In short, its going to cost. And with lower gears, you will want bigger tires and that means some type of lift. With small tires, you motor will be screaming when you are going 60mph. Good luck!
its rather easy until you need to set the ring and pinion
(and you´ll want to get it 100% right 
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