six inches
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For suspension lifts UP TO 6" total height increase the drive shafts need NOT be lengthened. Anything OVER 6" will or you run the risk of the slip shaft on the rear in particular running out too far and shearing off the ends of the splines.
Keep a close eye on the kit recommendations for axle shims though as it may be necessary to increase shim angle to avoid u-joint bind. This is dependent on the amount of articulation the lift will actually gain you. With most 4" lifts the factory 3º shims just need to be re-used. A 6" lift may need more than that. Consult the manufacturer of the lift.
Keep a close eye on the kit recommendations for axle shims though as it may be necessary to increase shim angle to avoid u-joint bind. This is dependent on the amount of articulation the lift will actually gain you. With most 4" lifts the factory 3º shims just need to be re-used. A 6" lift may need more than that. Consult the manufacturer of the lift.