this may sound stupid, but are you low on gas? i always try to think of the simple fixes first. my buddy's ram was braking very very poorly, and he was convinced it was the calipers...i checked under the hood, and his brake fluid resivoir was BONE DRY. we filled it, and like magic..better brakes.
if you have gas, then could be a lot of things...you could scan tool it at advance auto, see what the trouble codes tell you. that would be a good start.