Vibration between 50-65

[macdude443]

Where to begin.  I THINK I may have found the source but I want a second opinion.

My '86 wagon has 1980 front/rear axles installed to change the gear ratio from 2.35 to 3.08.  I did that last year.  It seems as though, ever since I bought the car back in 2014, it has has a slight vibration on the highway between 50-65.  The dash would wiggle up and down a bit, but nothing that was violent.  Most noticeable in my butt and down by my feet.  Hardly anything in the steering wheel.  A few weeks ago I thought I'd use a few days off to try and tackle the problem.

Up to this point, the car has had the following done with no change to the problem:

New tires
Axle change to 3.08 1980 units
Rear axle rebuilt (new bearings and seals, outer bearings re-packed)
All new U-joints
Changed to factory aluminum rims
Tires road-force balanced
New trans mount
No back end sag
Car has no lift.  Stock height, stock tire size.
258, rebuilt TF998 transmission, NP129 t-case
All new brakes and shocks

Since I had some time to mess around, I pulled the rear driveshaft and noted some vise marks, probably caused by myself as I changed the u-joints way back when.  I took it to get it balanced and for the heck of it, had them install brand new u-joints.

Install the balanced rear shaft and now I can't get on the highway and exceed 45 due to a bad groaning vibration in the same location, beneath my arse and in the floor boards.  I go home and notice that a shop previous had noticed the u-joint straps on the transfer case end were worn and had made shims to hold the u-joint tight.  They had fallen out, so I put them back in until I could get new straps.  Vibration was then pretty bad still but I could at least go 55 before it started.  I bought new straps for that yoke and put them in.  At that time I noticed that, before tightening them down, I could move the u-joint and it's caps back and forth slightly in the yoke.  Like there was extra space between the two locating tabs.  Can a yoke be bent or stretched?  Do they simply wear out?  Maybe the loose straps contributed to the wear.

Anyway, I pulled the front driveshaft and noticed no change.  Rules that out.  I also tried moving the u-joint around in the yoke and strapping it down in different positions and this seemed to change the speed at which the vibrations start.  Does it seem reasonable to assume that replacing the yoke with a better one should solve this issue once and for all?  On a scale of 1 beer to 10, how bad is it to replace a rear t-case yoke and seal?  If I can do it without an impact I'll do it myself.

[eaglebeek]

Did you pull the slip yoke apart? If you did check the alignment of the u-joints. U-joint crosses must be on the same plane at each end of the driveshaft. This image: http://avtoservizi.com/announcement/pictures/fb8f7edab3dc0a251a1ed5f28f0ed75a.jpg shows a driveshaft with the u-joints properly aligned.

[AMC of Houston]

Or somebody maybe installed the wrong U-joint with slightly smaller cap diameters??

[mudkicker715]

I have seen the yokes come off easily. Trying to get it off might show you it was loose in the first place. Maybe even a cause.

[85AmCfreak]

I had the exact same issue with my 83. Changed the t-case, differential yoke and had the driveshaft balanced. Vibration gone.

[macdude443]

The driveshafts are in-phase.  I made sure of that.

I used the u-joint called for by the book, so I'm almost sure it's the right one.  The previous one would have to have been wrong too, since I've now had two joints in every position.