99 XJ 4.5" RC X Series w/ tc drop all speed vibrations

Yes, before you even think of asking I searched for hours before asking.

I just finished installing my 4.5" Rough Country X-Series w/ drop brackets and full rear leafs this past week on my '99 XJ NP231 C8.25 AW4. I've only gone around the neighbourhood a few times (maxing out at 45 mph) but even with a TC drop in I've got terrible droning audible vibrations when on the throttle. It starts in at very low speeds which baffles me. I don't seem to have any excess radial play in the drive shaft at all.

I suspected worn u-joint crosses and replaced both of them with no change. I haven't thoroughly inspected my mounts yet but they didn't jump out as worn.

The RC leafs have an angle wedge preinstalled but I can't find anything on angle. So I know I should be under there with an angle gauge determining what angle things are at.

Now is it possible that rotating the drive shaft 180 deg. in the rear yoke could cause this issue? I reinstalled the yoke on the drive shaft as it came off.

I know I'll probably have to get a SYE but if I can defer doing that it will nuke my budget a little less. I've been looking at hack n tap SYE's and a stock front driveshaft as under 200 bucks total in Canada is better than 600 bucks with taxes and shipping for a HD SYE and custom driveshaft.

I realize that this guy is me Hasta , but the shop I order from in my province told me they've never heard of anyone having to use a SYE on a XJ which directly contradicts internet wisdom.

Check your rear pinion angle, with no sye it should be as close to parallel with the transfer case output at possible. If your saying the springs already came with degree shims that could be the problem. A degree wheel helps but its pretty obvious in the short distance between the two to tell what the pinion is doing

In internet terms it should look like this; output = driveshaft \\ pinion =....

If it looks like; output = driveshaft \\ pinion \\ there is your problem... haha

make sure the the thicker part of the degree shim is pointing towards the rear...in other words "fat to the back" haha. Anyways will roll the angle towards the t case, if you are using a t case drop I have seen where the shim will have to come out as well as a few cases where the shims ahd to ran backwards to correct. My advise if installed fat to the back and all angles are good, take the shaft to a shaft shop and have them check for balance

Thanks for the advice guys. I'm about 95% sure the shim skinny part is like an arrow pointing forward. I suspect that the shims are causing a double cardan type angle that's just not compatible. I expect I will just suck it up and order the SYE and fancy shaft.

Oh and Jason it's a bit unrelated, but I called about a week ago concerned about my shackles being so far forward that they were sitting on the edge of the pocket with the tires on the ground and a couple hundred pounds in the back. I called your customer service and was advised to put more weight and wheel it for a while.

I'd read about you installing the kit on three different Cherokees and getting the same results with a vertical shackle not a forward angle and the idea mine was different was illogical. So in a moment of genius of dumb luck I thought I'd throw a pry bar in there and see how much force it would take to flop them back a little and preload them that way. Well it turns out it required very little force and they popped backwards and are now vertical. I know it's hard to diagnose things over the phone or write idiot proof instructions, but maybe this will help someone else with the same problem.