Large lowering blocks can play hell with pinion angles and reaction distances, as well as movement. As the rear wraps up on launch, it can actually move rearwards- the pivot point is actually where it mounts to the spring, so when it tries to rotate it rotates around the axis of the mount; in this case the bottom of the lowering blocks; moving the whole axle housing rearwards. Along with the pinion rotation, this can effectively move your rear u joint backwards several inches. Housing movement can cause havoc with rear tire to wheel well clearance, especially if you factor in tire growth into the mix. Pinion/u joint movement can pull your slip yoke rearward, and if you have a couple inches of movement, I wouldn't trust slip joint engagement- especially if you get ANY amount of wheel hop. And the taller the blocks, the more exagerated the movement is.
