Here is what Ted says, so this affects 2011-2012
The EPA set new and stricter limits on NOX emissions from diesel engines starting in 2011. There were two ways that manufacturers could achieve the new targets. One was to introduce an SCR (Selective Catalytic Reducer) that used DEF (Diesel Exhaust fluid) to reduce NOX. The other method was to increase the amount of exhaust gas being fed back through the engine. This second method starves the engine of oxygen which then gets stripped from the NOX during combustion.
Ford and GM decided to go with DEF in 2011, while Dodge decided to increase the amount of exhaust gas recirculation. Dodge used this decision as a sales tactic against Ford and GM, appealing to buyers who were wary of the DEF based systems, but they had to switch to DEF in 2013 because emissions targets were once again made more strict by the EPA.