To reinforce the point that even Rawls' difference principle isn't really concerned with inequality at all, look at this excerpt from Leif Wenar's SEP entry.
The second part of the second principle is the difference principle. The difference principle requires that social institutions be arranged so that inequalities of wealth and income work to the advantage of those who will be worst off. Starting from an imagined baseline of equality, a greater total product can be generated by allowing inequalities in wages and salaries: higher wages can cover the costs of training and education, for example, and can provide incentives to fill jobs that are more in demand. The difference principle requires that inequalities which increase the total product be to everyone's advantage, and specifically to the greatest advantage of those advantaged least.
Consider four hypothetical economic structures A-D, and the lifetime-average levels of income these would produce for representative members of three different groups:
Economy Least-Advantaged Group Middle Group Most-Advantaged Group A 10,000 10,000 10,000 B 12,000 15,000 20,000 C 20,000 30,000 50,000 D 17,000 50,000 100,000
Here the difference principle selects Economy C, because it contains the distribution where the least-advantaged group does best. Inequalities in C are to everyone's advantage relative to an equal division (Economy A), and a more equal division (Economy B). But the difference principle does not allow the rich to get richer at the expense of the poor (Economy D). The difference principle embodies equality-based reciprocity: from an egalitarian baseline it requires inequalities that are good for all, and particularly for the worst-off.
Pay special attention to Wenar's illustration with the chart. To correctly apply the rule you never have to look at the last two columns. That's because they are irrelevant to the actual underlying principle obscured by the equality-centric language of the difference principle. (The last two columns are very relevant when we start thinking about the strains of commitment and stability in a dynamic context, but that's a different issue.) The idea of inequality here is an idle conceptual gear that catches on nothing and does no intellectual work. Rawls' real rule, cleared of distracting superfluities, seems to be: find the system that leaves the least-advantaged best off and pick it.
Rawlsians: what say you?