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I don't see it really affecting shackle angle since the shackle is ever-changing as the car goes down the road from bouncing or from acceleration or deceleration.
How is it taking weight off the rear tires if anything you have just added the additional weight of the spacer to the total weight of the car. What the spacer has done is make the distance between the front wheels and the back wheels greater which should make it handle better. A longer wheelbase always handles better than a shorter wheelbase.
I could see where that driveshaft being out an extra half an inch might cause a vibration. You just have to drive it and see.
Jmho. That as you know I highly value.
