I've been trying to track down some driveline vibrations that I've had for a while that got worse when I recently did a 2" lift on the rear. I'm posting this in the stock XJ forum since the problem started long before I did the lift. I get the typical vibrations from the driveline under acceleration a bit around 35-40 mph and then more pronounced around 75-80 mph. Sometimes at neutral throttle in the 75-80 range I'll hear and feel a bit of a rattle/thumping as well. I replaced the rear driveshaft u-joints, but that didn't help. I've greased the slip yoke, and that didn't help. I checked my pinion angle a few years ago when it first started, and I don't recall the exact number, but it was fairly low and within spec and the TC output shaft and rear diff pinion were parallel, which is good. I removed the rear driveshaft, and that made it go away, so it's something along that route. It sounds like it's coming from the general area around the slip yoke, and not the rear diff or anything like that. Like I said, the 2" lift made it worse, but it's been there all along. The exhaust is not hitting the crossmember.

I'm starting to wonder of the output shaft bearing on the transfer case might be the problem. Here's a video I made tonight showing me grabbing the slip yoke and trying to shake the whole thing side to side and up and down. Yes, you can see the TC move around a bit as the trans mount flexes (normal), but what you may not be able to see very well (but certainly can hear) is some small amount of play in the the tail shaft. It's not much, but it's enough to get me thinking it might be part of the problem. I tried the same thing on the diff end and got zero movement out of the pinion.

Any thoughts on this situation? It seems ridiculous to put a SYE on a 2" lift, so I'm hoping I can sort this out.

 

Pull the driveshaft off, then try it. See if it's the output bearing or the slipjoint splines. I'll bet it's the splines.

 

Yeah, I was thinking the same would be helpful, but I didn't feel like getting greasy at 11:00 last night. Will to today and report back.

 

That yoke looks far enough out that it is a good possibility it hasn't enough of the splines engaged to keep it straight

 

That thought also occurred to me, as it definitely gets worse the higher the rear gets. I figured a 2" lift shouldn't pull yoke out far enough to really be a factor, but who knows what else is going on. I'll snap a pic with the boot off and post it up.

 

I dropped the driveshaft to take a closer look, and here are some details along with pics, videos, and measurements. The first two pictures show the driveshaft in place with the grease boot removed so you can see how the slip yoke engages the splines. These were taken on level ground with a half tank of gas and nothing in the back of the Jeep. The total length of the splines on the TC output shaft is 3 1/4 inches, and the part you see not engaged measures 15/16". That means at rest about 71% of the splinted area is engaged with the slip yoke. For reference, I'm at 19 1/4" between the center of the hub and the bottom of the rear fender flare.




Here's a crappy pic of my driveshaft just for reference:


I also measured my driveshaft and pinion angles with an angle gauge to check if that might be a contributing factor. The TC output and rear diff yoke appear to be parallel, so that's good. Not sure how the driveshaft angle relative to the TC and diff yoke falls on the problematic scale, so I'd be curious to hear your thoughts.

Transfer case output shaft angle: 4°
Rear diff yoke angle: 4°
Rear driveshaft angle: 15°

Finally, I took some video of me shaking the slip yoke with the boot removed and shaking just the output shaft. I still get a little bit of knocking when I shake just the TC output shaft, but the majority of the movement and sound comes from the slip yoke on the splined shaft. If I slide the slip yoke further out the shaft it gets worse, and if I slide up more than it normally sits it gets better.


What are my options here? I'm not lowering the rear suspension, and a transfer case drop IMO is a just a band-aid fix (and looks and performs off-road like crap). Do I need a longer driveshaft? Do they make longer slip yokes? If I'm going that route, should I just do a SYE? That seems like overkill for my rear ride height.

 

The best option is a slip yoke eliminator and double cardan driveline. Prices vary but you will be glad you did it

 

I was afraid you'd say that, but it might be inevitable. I should add my recent 2" lift is on top of whatever my 5-year-pld Dorman 929-301 leafs gave me.

Also, I get how a SYE works and how a double cardan driveshaft works. I also get how you have to shim the rear axle so the diff yoke points straight at the TC yoke. What I don't get is how the lower shock mounts fit into the hole in the bottom of the shock with the axle angled up. Any pointers?

 

With a 2" lift, you won't have to shim it up too much. The lower bushings are forgiving. But if you want to correct them, there are universal shock studs that will mimic the ones you have and bolt on after you change the bracket

 

I just ordered a SYE and rear driveshaft from Tom Woods.

 

Can't go wrong with Tom Woods. I have bought three driveshafts and a parking brake for my tcase from him. Great stuff