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There is plenty of lifted Broncos with 6 inch lifts. You shouldn't need to lengthen anything at 6 inches for a street truck, but if you actually use the full motion of your suspension I'd invest in some extended brake lines. The drive shafts should be alright.
i wheel my bronco alot and the brake lines do not need any attention with a 6" lift. i have cycled my suspension and have found the shocks limit me more than anything.
Like Edgethis said, driveshafts will be fine
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.