They dropped the Ford 3.7L n/a V6 in favor of the smaller and less powerful n/a 3.5L that they use in their midsize cars.
With the weight reduction of the 2015, ford reduced GVWRs and bigger engines.
6.2L gone and is now replaced by newer 385 hp 5.0L
3.5L turbo Ecoboost is carried over with same power output
Previous lower 360 hp 5.0L spot is now replaced with the 2.7L turbo Ecoboost
3.7L gone and is now replaced by less powerful 3.5L n/a engine
Still 4 available engines, just stepped down in power output
With the lighter weight, and the new 385 hp 5.0L, the 2015 will be a very quick truck.