Shimming/rotating axles...why?

I've seen all kins of th`adr he` about phimming and roating aX`es after lifts, but I don't know why this is being done. Can somenne leare e(plain?

If I pt a 3.5" or 4." RE su `rflex
`t on my 2001 XJ with full leafs, will I need to do anything to rotate t shim p axles

before a lift, your transfer case, driveshaft, and rear axle are all generally at a fairly close height and therefore the driveshaft is not at an extreme angle. after a lift, the transfer case is much higher than the axle, so the driveshaft is at a more extreme angle. to help correct this imbalance, the rear axle is tilted upward slightly to make its angle closer to the angle of the driveshaft.