If you have regular U-joints on both ends of the drive shaft, then the rear axle shims should be adjusted such that the angle at both ends of the driveshaft is the same, otherwise you will have different rotational velocities. If that doesn't fix our vibration, then rotating the rear pig up will only cause exzcessive wear.
I'd tend to believe the guys here on the forum, not some salesman at a mail order house.
It may be that the lift you installed caused the maximum U-joint angles to be exceeded, causing the vibration. If you currently have a slip yoke, I'd first install a SLE kit -- keep in mind that the reason for doing this is to increase the effective length of the driveshaft. Increasing the distance between the front and rear u-joints will result in lower u-joint angles. After doibg that, ensure that both u-joins are operating at the same angle. That may be enough to stop the vibration. If it still vibrates after installing the SLE kit, then I'd recommend a double-cardan (CV) joint at the front end of the rear driveshaft. Now, with a CV joint, you don't ry to achieve equal angles at both ends of the driveshaft. You adjust the pig such that the pinion points directly at the CV joint. Actually, most people point it one or two degrees off when sitting static. That way, when normal torque is applied to the driveshaft during vehicle operation, it will torque the pig up a little bit, so it's pointing at the front u-joint in a dynamic condition.
1977 CJ-7, fiberglass body, AMC 360 w/ headers, DUI ignition, Edelbrock intake and Holley 4150 carb, TF999, Dana 300, 4.56 gears lockers, York air comp, 4" susp lift, 2" body lift, BFG 35" M-T tires, Megashifter, AGR pump & box, REP8000 winch.
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