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This is a discussion on Vibration/Shuddering after 8" lift install within the Offroad forums, part of the General Discussion category; pictures pictures pictures...
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1999 ECSB Seirra 5.3/4L60E/NP241 SAS 60/14ff 5.13's/40's
2011 Chevy 2500 HD 6.6 Dmax/Allison LTZ CCSB Z71 35" MTZ's
when you go over 4 inch suspension, you are supposed to get your drive shaft lengthened. i put a 6 inch on mine and i had to lengthen mine 2 inches.
01 SFA 1500: project truck, in pieces again
87 Jeep Cherokee: trail rig, slowly getting built
03 Buick LeSabre: fully loaded daily driver
84 M1009: constantly broken money pit, finally gone
Post some pics of your rear driveshaft angles. What you are describing sounds like a driveline vibration, more than an alignment issue. How was the back end lifted? blocks? Keep in mind that the working angle on a standard u-joint in only 2-3 degrees. By that I mean that the angle on the front u-joint and the angle on the rear u-joint have to be within 2 degrees of each other, or you will get vibration. Ideally, the rear u-joint should be at an angle 2 degrees steeper than the front u-joint to compensate for axle wrap. This way the angles will match each other when driving. If your angles do match each other, and you aren't getting excessive axle wrap, then you might want to invest in a cv shaft. I went throught the exact same thing when I lifted my k5 only 4 inches (shorter wheelbase makes the angles alot worse). It would vibrate slighly when accelerating hard at slower speeds, and on the highway it had a really bad vibration coming through the floor (like driving on a rumble strip). Rather than just throwing in a shim, measure the angles first to see how many degrees (if any) it needs to be shimmed. If the angle at the transfer case is greater than the angle at the diff, shimming will make it worse.
To increase the angle at the diff, you can either switch to different blocks, or use shims to point the pinion down. Do not just leave it! Driveshaft vibrations can destroy u-joints, pinion bearings, and even cause damage to the transfer case. I have seen people actually crack the tail cone on the transefer case in some cases. Before buying shims, measure your angles to see how much you need, as shims come in all different sizes.
Turning the blocks around would most likely point the pinion down too far.