2004 Mercury Mountaineer Differential broken--Please help!

Our car was taken in recently due to an odd noise during acceleration. The alignment was done in addition to new tire bearrings.
About a week later the car made a loud noise which turned into a grinding noise. The car then started smoking and would not move. A local mechanic looked at it and said it looked like the front axle was damaged due to the poor alignment.
My questions are: is this a correct assessmant, how difficult/costly is the repair and how can I avoid this problem in the future.
Any assistance you can provide will be greatly appreciated.