Lowering blocks raise the diff up into the car (add space between the diff housing and leaf spring) Lowering blocks are ok for a roadgoing 'driving miss daisy' car, if the car is to be used for anything else then i would suggest getting the leaves reset. When you accelerate hard through/out of corners with a decent amount of power the diff twists up and can cause axle tramp, this will only get worse with lowering blocks as you can imagine.