There are too many topics on this, so I thought I would present my resolution for critics.

I have late model 96 XJ with AW4, NP231J, 3.5" RE Superflex lift with 147k. Wheels are new 15x8 with 3.75" backspacing. Firestone destination M/T with decent tread (looks pretty even from left to right). PSI is set to 30 all around.

I'm getting a pretty rough vibration over 55mph which goes away as soon as I let off the gas. I have not tried going faster than 60mph because it just gets worse.

The front pinion seal is leaking, but not severely. Plenty of fluid in the D30 transfer case. When I put the auto trans in gear it makes a slight clunk (which makes me believe it is a drive shaft issue).

My thoughts on the vibrations are:
#1 - Bad front drive shaft U-joints (probably original)
#2 - Bad front axle shaft U-joints (also probably original)

I had a front end alignment and tire balance done at Les Schwab after I installed a Rusty's HD drag link/tie road combo. These idiots are telling my front LCAs bushing are shot (axle side), but they are flex joints not bushings.

Any thoughts would be appreciated.

 

Do U joints and the seal. If the seal blows again it's the bearings behind it wearing the seal.

Don't forget that ****ty tire balancing will cause this.

ARE the joints shot? I assume the LCAs are new, torqued and greased properly.

 

^^^^^
I believe this is part of the problem along with the alignment. I might try another shop before I totally discount it as the problem.

I need to get under the Jeep in between snow storms and check to see if the joints are actually shot. If they are, I will replace them along with the seal.

The LCAs are not new, but are in good shape. Weird part is I called Rubicon Express, and they told me to torque bolts on the front axle and make sure to pinch the ends of the LCA bracket onto the flex joints. Wouldn't that defeat the purpose of allowing the flex joint to flex left and right?

 

You have these guys right?



The sleeve, of both the rubber bushing and the flex end, are meant to be completely clamped. They do not rotate when the suspension works. At the axle end, the flex joint will rotate internally. The bracket shouldn't be able to compress against the arm itself.

 

They look similar except mine are adjustable.

 

The rubber and the joint itself are what flex. The inner sleeve should be clamped tight. Torque on LCA's is 85 ft-lbs. Only tighten rubber bushings when the weight of the vehicle is on the tires after bouncing the suspension.

You can test the u-joints by seeing if there is any play in them. If it is bad you may see where they have been making rust, or you can move them with your hands, or with a pry bar under the caps.

You should really check out the entire front end. See if the track bar is loose. See if the ball joints are loose. Make sure the wheel bearings are good. Make sure the steering linkage is tight.

You can remove the front driveshaft and see if the u-joints or double cardan ball is shot. You can even drive the vehicle without the front driveshaft to see if the problem goes away.

While the driveshaft is out, you can turn the pinion by hand and see if it is smooth. If it is rough then you have pinion bearing problem. IF you end up removing both axle shafts, you can also remove the differential carrier and see if those bearings are good.

I am going to guess your problem is a lot simpler than the differential bearings. There is only so much that can go wrong in the front end.

The clunk when you put it in to gear is probably the rear driveshaft slip joint. Remove the driveshaft and grease the splines. Any old grease will work, like you'd get in a 1 pound tub. Some people use anti-seize. In some cases, a problem in the rear differential case also cause clunking, like excessive backlash. Unless you are in 4wd, the front driveshaft just goes along for the ride and turns freely.