You're going to have an awesome trip. I am crying a little with envy.
When I said "hardtail" I didn't mean to imply a MTB with a suspension fork. As pointed out, you want a fully rigid MTB, the kind no-one bought after about 1995. Because you will need a front rack with low-rider panniers, and that needs to go on a rigid fork. Carrying a touring load (>40 lb) in just rear panniers will make the bike an incredible pig (I have done that), so distributing the weight is pretty much necessary. Also fenders because you'll be riding in rain and mud sometimes.
By the way, I know that coming from a backpacking background, you are used to carrying 50+ lb on your back. You might be thinking, hey, I can carry lots more on a bike, it's like a mule with wheels! My backingpacking buddy and I made that mistake on our first mountain bike camping trip. Turns out that 50 lbs on a bike is a lot of weight, because you have to pedal and steer and balance that mule. Gram-paring will be just a critical on the bike as on your feet.
Include bright lights and lots of reflective tape on panniers and clothes and helmet, I don't think you will find "bike lanes" where you're going. I would build a front wheel with a generator hub to power those lights. I would use the stock rear wheel, take it apart and rebuild it true and strong. Road tires, not too fat.
Ideally I would use wide drop bars, but the simpler alternative would be bar ends. It is not so much that you need the aero position of drop bars, not with 70 lb of bulging bags, but that you need multiple hand positions or your wrists will scream after hours of riding.
You sound pretty knowledgeable about bike maintenance and repair. I would do an complete overhaul on the bike you buy, do it by yourself, whether it needs it or not. Simply to make sure your hands can fix the bike out there on the road, and that your tool kit has what you'll need. If you started a "what tools should I bring for a two year bike touring trip in South America" I think everyone would have a lot of fun, just don't listen to every reply or you will be pulling a trailer for the toolkit.
Anyway, the bike. Something like:
http://atlanta.craigslist.org/nat/bik/3382857156.html
Roughly 1990, a Bridgestone MB-3. There is no more perfect bike for your needs. Top quality brand, old-school rigid MTB, not too light (read: it will be sturdy), simple thumbshifters, the gearing and brakes will be perfect as is, all you'll need to do is swap tires, add lights, racks, fenders, and bar-ends. Can't see in the pics but I am sure it has fork eyelets. One set of eyelets is enough, you can use the same eyelet for rack and fender. Only $125.
And, not as perfect but I think they'd do:
http://atlanta.craigslist.org/atl/bik/3445293132.html
Raleigh $65. I'd swap the trigger shifters for thumbshifters.
http://atlanta.craigslist.org/nat/bik/3448444976.html
Trek 820 $100
http://atlanta.craigslist.org/nat/bik/3378166265.html
Specialized Rockhopper $150 which is $50 too much but I imaging they'll dicker.
http://atlanta.craigslist.org/atl/bik/3421091704.html
Fisher $195 Seems overpriced by 2X, and I would change the gripshifts for thumbshifters, because when gripshifts start binding and jamming, they are hard to repair.
Finally, I wouldn't want too pretty a bike. Not that you should permanently deface that nice Bridgestone, but maybe wrap the tubes in black electrical tape or something. And then I'd lock the hell out of the bike. Where you're going, that bike will be your car and your house and your biggest possession.