Some go 4" and have no problems and some go 2.5" and have driveline vibes. Depends on the brand of lift, your Jeep and how the pinion angle settles in. It also has to do with the wedge on the rear springs and how manyd egrees it moves the pinion. To get some info on pinion angle which is the big deal with a lift you can do a lot of reading here halfway down right side grey bar Technical Issues.
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