The old adage that teams should trade players to teams outside their division seems to be followed by NFL teams. The data used are the 275 traded NFL players from 2010–2015 (from http://www.foxsports.com/nfl/transactions).
Of the 275 traded players, only three were traded within the same division. Two of those players were traded from the San Francisco 49ers—Kentwan Balmer (to Seattle) and Isaac Bruce (to St. Louis). Donovan McNabb (from Philly to Washington) was the only other player traded to a team in the same division. Curiously, all three of these trades took place in 2010.
It doesn't look like particular divisions trade to any other particular division. The distribution of trades seems to be equally spread out to the other divisions. It also appears as though divisions are trading players away at about the same rate as they are acquiring players (rather than trading players for draft picks).