Why need shims with H&T?

I'm curious why one would need shims for the rear axle if they did the hack and tap. Wouldnt the hack and tap eliminate all vibes or does it depend on the amount of lift?

Because with a CV shaft you have to rotate the axle up to meet the driveshaft angle.

RCP Phx said:

Because with a CV shaft you have to rotate the axle up to meet the driveshaft angle.

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Ok that would be either an after market CV shaft or a front XJ driveshaft?

Either one.

A stock XJ rear driveshaft has a single u-joint at each end. They're phased so that when the operating angle at each is equal, there are no vibrations produced.

A driveshaft with a constant velocity u-joint at one end and a single u-joint at the other end requires that the operating angle at the single u-joint be nearly straight.

Why? When the operating angle of a single u-joint is not straight, assuming a constant rotational speed on the input shaft, the speed on the output shaft varies in a sine wave, twice per revolution. Okay, it's actually a cosine relationship, but the idea is that the speed of input to output is *NOT* equal. On a standard driveshaft, the sine waves cancel each other out, returning to a constant speed without vibrations. With a C/V jointed driveshaft, the single u-joint needs to vary in speed as little as possible.