Going Higher, Could driveline vibes get worse?
 

So currently I have a 3" Trailmaster kit. It was an AAL kit so naturally the old leafs that were already wore out weren't much good anymore. Replaced the rear packs with 3.5" ProComp leaf packs and that's when the driveline vibes started. I added a .75" spacer to the front to level it out and added a 1" t case drop. The vibes are much better with the t case drop but they are still there.

I decided to go a little higher by adding .75" Rusty's shackles in the rear and replacing the front .75" spacers with 1.75" spacers. So I'll be at roughly 4.25" all around. I don't believe the front springs are a true 3" lift, more like 2.5".

With that being said, since adding the shackles doesn't necessarily move the axle down more but lift the body up some, would this make my driveline vibes worse? And what about adding the taller spacer in the front?

I've looked at doing a SYE but since mine has a 242 t case my options are kinda limited to a Hack N' Tap then adding a front driveshaft to the rear for that. I'd like to do this and get rid of the t case drop all together but I'm rather skeptical about doing this method for a SYE.

I'm replacing my shocks with 4"-6" Skyjackers Hydros and extending my rear brake line while I'm at it since it's pretty much at its limit with the new leaf packs. Is there anything else I should be aware of?

I'm still new to this so take it for what it's worth. But it shouldn't matter if you are lowering the axle or raising the body, it is still increasing the height difference.
From your past experience of vibes getting worse after increasing the height, I don't see why this would be any different.

I was under the impression that putting the taller springs in the rear moved the axle down causing the vibes to start. Putting taller shackles would moved the body up keeping the driveline the same. I'm probably wrong though. I'm in the process of trying to figure out what I need to do to get the Hack N' Tap done so I won't have to worry about it period.

Ok stop hold on please. There's no lower of axles, raising of body. The xj doesn't have a body to raise per say. Anything you add to the suspension raises the chassis of the jeep and ALL driveline components attached to it. The ONE thing that extended shackles does is raise the pinion angle of the rear axle, the raise depends on several factors. To stop the vibes properly you'll need to get an SYE. The 242 has the HnT or the Tom Woods, iirc, one. Google it. And it doesn't matter what you do to raise the jeeps height, minus tires of course, you'll have items that should get changed too. CAs, TB, Brake Lines, SYE, etc. There's a chart stickied for you to view on that matter.

Keep in mind, lift shackles will rotate the axle, causing the pinion to be at a steeper angle. This needs to be considered for proper driveline geometry.

Stock 2-joint driveshaft requires the angle at transfer case and the angle at rear axle to be equal.

Double cardon drive shaft (SYE) requires most all of the angle to be at transfer case end where the double joint is. The axle pinion to driveshaft should be near zero angle.

I suggest studying driveline geometry and understanding the mechanics. Then you can look at your jeep with a much better understanding and eliminate guesswork. Drivelines are either correct, or they are not. Very little wiggle room.
http://www.fourwheeler.com/how-to/tr...veshaft-angle/
http://www.hotrodhotline.com/md/html..._harmonics.php
http://www.4xshaft.com/driveline101.asp

Ok, well I'm gathering info now on how to do a Hack n' Tap now so I can do I can do it correctly so I won't have to worry about vibes anymore regardless. More or less I'm unsure of how much worse adding these components will make the vibes now. The t case drop helped but they are still present slightly.

However, I was unaware of what adding shackles would do exactly, another reason I asked the question.

Thanks, I'll give these a look and see what I need to do to do this correctly.

Were here to help out and are happy to do it. We get a little worried when someone already has a wrong idea in their head because they sometimes dont trust us. We want the best for you and your jeep and as long as you listen to our constructive criticism and advice, we got you.

One thing I'm curious about is the new springs came with shims. I believe they are 4* shims. Can those face either way to pitch the axle up and down? The thick side is towards the rear at the moment. I'm gonna get underneath when I get a chance and see what the angles look like.

If the stock rear drive shaft is still being used, the 4 degree shims are likely the main cause of vibes.
With those shims the axle pinion is probably pointing at the transfer case, this is correct for a double cardan, but will cause the stock system to be out of phase.
U-joints must be 'in phase' to eliminate vibrations.

Yup.

Maybe a little visual will help also.Driveshaft angles and phasing

Thanks. That makes it more clear to what could be going on.

Yeah I'm still using the factory driveshaft. The shims have the thick end facing the rear so that would make the pinion shift down correct? If I turn them around to where the thick end is towards the front then that would pitch the pinion angle upwards. Or I could just take them out to see if that helps. Since you say they may be the main cause of the vibes I'll take them out when I get a chance and see if anything changes.