Thank you for the advice, however it does not answer my question of what the differences are.
I think the common answer will be, "it depends". It really does depend on your uses. Though each generation seems to lend itself towards a pretty good mix of on-road and off-road use.
For a good, tough, truck frame rig that's capable of conquering the rubicon, but that has PRETTY good road manners = Gen II
For a comfy, quiet rig that gobbles the road miles, and then will be able to scramble up the rutted, washed out rock garden to get up to a solitude-infused fishing spot. = Gen II or Gen III
For a long-hauler that only see's dirt roads occasionally, and in those instances those roads are graded, but of questionable traction, definitely the Gen III
Don't forget the bulge-fendered Gen II.5's, either. Some think that they're the best of both worlds. Comfier, plusher than the gen II's, but with a more sure-footed 4x4 system complete with rear locker if you get the winter package. These ones still have a truck frame, I think whereas the Gen III's are unibody....