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Give or take, 8". But there's no one formula that fits every SD configuration. Some have had no problems running the stock driveshaft at 8", while others have had to get a new CV style driveshaft at 6". The determining factor will be vibration that can't be taken care of by shimming the center bearing (assuming you have a 2-piece driveshaft) and/or tilting the rear pinion with angled shims under the rear springs.
Fordlovin,
I have about 10" of lift on a SCSB. I did not have to extend the driveshaft but the u-joints wore out quick. My recommendation is if you lift more than 4" you should really purchase a new shaft with the "CV" joint (formerly known as a double cardan). I purchased mine from arizonadrivelines.com in Phoenix for about $450 delivered. It might sound like alot of money, but it will save your drivetrain from damaging vibrations. Just my $.02.
I had a CCSB w/ 6" and needed no driveline mods. I've also done several 6" w/o the need to modify, but as mentioned it really depends on the truck, some are just picky.
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