I have a 99 Cherokee Sport that I recently put a 3.5" Rubicon Express Super Ride lift into. Initially, I was experiencing some vibes on acceleration and a loud rattling noise when I would take my foot off the gas at speeds of 30+. I dropped the transfer case 3/4" and shimmed the rear axle 6 degrees. For the most part, it seems to be a pretty good combination. The majority of the vibes have been eliminated and it feels pretty good. However, that loud rattle (almost like a grinding sound but I have checked everything under the jeep and there is no visible binding anywhere so I think rattle is a better description) still exists. It doesn't always occur but does regularly. I had hoped that dropping the TC and shimming the axle would eliminate the noise too, but it hasn't.

I'm hoping somebody reading this has experienced the same thing and might be able to help me out. Any ideas, suggestions, etc would be appreciated. Thanks.

Heat shield on the exhaust, maybe?

Though dangerous, I've heard of a guy that put his vehicle up on jackstands to reproduce a vibration; he was able to look underneath (from the side) and locate the noise.

I believe you still have a U-joint binding situation. A 3/4" drop is not enough and the rear shim might be too much in angle.

IMO, you need a SYE system (do a web search: "modified Jeep SYE" and you will get all variations).

I appreciate the feedback from both of you. It's definitely not the heat shield though given the amount of info I provided that could have been a possibility. What I should have mentioned initially was that the vibe/loud noise sounds like it is coming from the realm of the transfer case and when it happens I can feel it in the floor. My gut tells me that something is momentarily binding when I take my foot off the gas, but I have checked the u-joints and there is no visible evidence of abnormal contact. I have been driving the jeep for about a month now so I would think that if the u-joint was binding there would be some obvious visible indications, but there isn't.

Today I lowered the TC another 1/4" to make it a full 1" drop. This seems to have helped a bit, but definitely has not resolved the problem. Alohabra, I have been considering going with an SYE and CV driveshaft and have heard that it will solve my vibe problems but as of yet I have been trying to avoid dropping the extra cash. Money is tight so I would like to try to dial this in without going that route. In your opinion, do you think I am wasting my time messing with shims and TC drops entirely?

IMO...yes.

Note:
My lift is only two inches, but my TC is lowered 1 5/8" with a spacer cut from "superstrut". I think you have too much angle in the shim...take it out completely. Draw a picture of the TC drop and the angle of the pinion and you will see why. Both ends of the drive shaft need to be at the same angle within two degrees, when you don't use a CV driveshaft. With a CV drive shaft you use a shim but not a TC drop.

Search for other people's opinions on this one, but a 3 1/2" lift is too extreme to solve without a longer driveshaft anyway. To keep the same drive shaft angle as original with a 3.5" lift theoretically you would need a 3.5" TC drop!

IMO...yes.

Note:
My lift is only two inches, but my TC is lowered 1 5/8" with a spacer cut from "superstrut". I think you have too much angle in the shim...take it out completely. Draw a picture of the TC drop and the angle of the pinion and you will see why. Both ends of the drive shaft need to be at the same angle within two degrees, when you don't use a CV driveshaft. With a CV drive shaft you use a shim but not a TC drop.

Search for other people's opinions on this one, but a 3 1/2" lift is too extreme to solve without a longer driveshaft anyway. To keep the same drive shaft angle as original with a 3.5" lift theoretically you would need a 3.5" TC drop!

Im in agreement with Aloha.
3.5" of lift is alot for a post 95 XJ. You didnt mention if you have full leaf packs or an AAL. If you have full packs, I can almost guarantee you have more than 3.5", probably closer to 4.5".
I think you are experiencing vibes, whether you feel them or not. They will sometimes sound like a groan. Im also in agreement that you are probably not using the shims correctly. Even if you do have the angles dialed in, how much of the slip yoke is still in contact with the T-case splines? I'll bet you can crawl under and shake the driveshft up and down on the splines by hand.

Just to add to Ray's post...

Take the rubber boot off of the slip yoke and you will see where the problem is...too much lift makes the yoke pull out a lot further than stock..

That is why they use a SYE. The slip yoke splines are where the vibes are...not the u-joints as I previous said...

I do have full leaf packs in the back, not add-a-leafs. Ray, you are right. I can shake the driveshaft up and down a bit and I suspect that all of you guys are right about the origin of the noise and vibrations being the slip yoke splines. Before I go the CV driveshaft & SYE route, I think I owe it to myself to try to match the correct angles to see if it will work. My cousin has an '01 with the same lift and he was able to drop the TC 1" and shim the rear axle ever so slightly to make it work. That said, I know every Jeep is different but I think there is still hope. If I can match the angles and it still doesn't feel right then I will definitely go the SYE & CV route. Ray, I have read previous posts where you have backed the RE Hack & Tap system. For a rig that primarily serves as a daily driver, do you think that would be a good solution for me?

Some people say the H & T is weak. My feeling is that it is as strong as the stock transfer case mainshaft because it utilizes the stock shaft. So, by using a H&T, you dont really gain any strength but you do gain some driveshaft length (which decreases the angle) and a fixed yoke on the tcase. I have yet to hear of a H&T failure. Ive got one in my YJ that has worked flawlessly for over seven years and I havent babied it. I actually broke the tcase main shaft a couple years ago and the H&T itself lived on and I installed it in the replacement tcase. So if you are going to have a failure, it will most likely be the stock tcase parts and not the H&T parts.
Im in the process of lifting my XJ 3.5" and Im putting a H&T and an XJ front driveshaft in place of the stock rear shaft. You should look into that. Its a cheap way to get a SYE and CV shaft for the rear.
Here is what you need to do the front driveshaft conversion.
A front driveshaft from a 4.0, 4X4 automatic,
An RE H&T kit,
and Spicer adaptor pn 211229x
http://www.rockforddriveline.com/Shopping/R2-21-1229.asp

I paid a total of about $100 for mine.
The H&T was given to me, so unless someone gives you one, it will cost you about $100 more.
I paid $60 for the Spicer driveshaft adaptor.
I paid $40 for a used driveshaft.
Im putting all new ujoints in the driveshaft so I figure another $40 for those.