[ramoine]

Anytime you raise up the truck , you are changing the cv joint angles . By lowering the diff with that kit , they put those angles back very close to factory specs .
If you don't lower the dif , then the angles created from the lift will over time wear out the cv joint boots and cause them to leak . This also puts alot of stress on the cv joints which over time could cause them to fail .

Hope this helps ?

Bill.