Body-shaking vibration when I decelerate

[Icantdrive55]

When I remove my foot from the accelerator, my '86, FJ-60 makes a noticeable and resonating vibration that shakes the whole truck. It generally occurs at decelaration from about 65-70mph or when I'm going down a hill, when I habitually feather the accelerator to regulate my speed regardless of the slope, and rattles the whole truck to the extent that I can feel it through the back of the driver's seat-- but no shimmy in the steering wheel. The truck has always had a certain looseness or slop in the driveline; if I'm not careful to feather the clutch release, the truck starts off in a herky-jerky motion. Yesterday, I was again nosing around under the truck attempting to discover the source of the vibration. When I grasped the rear drive shaft, there is noticeable free-play of about 5-10 degrees in the shaft rotation. Is this normal for this truck? Has anyone else encountered these symptoms?
Is this a matter of driveshaft balance?
Perhaps a mismatch between engine speed and rear wheel rotation speed?
The truck has 217K on it and has all original equipment.
Thank you in advance for your comments.

 

[Wile E Coyote]

Drive shaft out of balance.
U joints.
Drive shaft slip joint splines worn.
Loosey goosey drive shaft flange (big loss of preload or bad pinion angle from severely sagged springs)
Male pattern baldness or overdue cellphone bill

 

[TobyB]

What Wile E said. If you attempt to move the rear driveshaft up and down where it comes out of the t- case,

can you?

That'll do what you describe in a heartbeat.

t

 

[Icantdrive55]

Drive shaft out of balance.
U joints.
Drive shaft slip joint splines worn.
Loosey goosey drive shaft flange (big loss of preload or bad pinion angle from severely sagged springs)
Male pattern baldness or overdue cellphone bill

Replaced the U-joints recently but that didn't help. When I had the drive shaft on the garage floor, I examined the slip joint and it didn't appear worn but, then again, I've never seen one that doesn't have 200K miles on it.
And, yes, there is some male pattern baldness going on.

 

[Icantdrive55]

What Wile E said. If you attempt to move the rear driveshaft up and down where it comes out of the t- case,

can you?

That'll do what you describe in a heartbeat.

t

Tried to move it up and down, back and forth, etc. No side to side motion--only that 5-10 degree motion on the rotational axis.

Another, possibly related, symptom I neglected to mention is a pattern of oil splatter on the underside of the body directly above the front of the t-case. I know there is a bearing at that point and the fact that it is leaking probably also means that the race and bearings have worn in addition to the ruined oil seal. Perhaps at highway speeds this wear imposes much more side to side movement than I can just by grabbing and shaking with my bare hands. Any speculation on that new information?

 

[Wile E Coyote]

When the driveshaft was out, did you add any grease to the slip joint? Add 6 or so pumps from the grease gun to the zerk on the shaft. Don't overfill the cavity with grease, that can crack the t-case, no joking.
5-10 degrees of rotational freeplay sounds normal, so no worries there. The t-case flange sounds like it needs looked at, but if you've recently repacked the u-joints (or installed new ones) some excess grease should be expected to fling up onto the underside. Side to side, up or down, or in or out slop and/or "notchy" feeling rotation and leaky seals are clear signs for sure. Plus, the front output flange doesn't come into play with the t-case in 2hi.

Lock it into 4hi, remove the rear driveshaft and get it up to 50-60 on a smooth road, the vibes should be gone. With the drive shaft off, put an angle finder on the face of the t-case and rear diff flanges and note the angles, if they're off by greater than 2 degrees, then that's the problem.
Look here for more details: http://www.4crawler.com/4x4/CheapTricks/Driveline-101.shtml
If the flanges are all tight and smooth, the angles are acceptable and the tires are balanced, then I'd blame the driveshaft (look for dents or missing balancing weight), but I'd expect the vibes to be present most of the time instead of only on decel.

 

[lcwizard]

Check to see if your driveshaft is in phase. Imagine the drive shaft without the u-joints and flanges.. the ears on the yokes at each end of the driveshaft main body should be in line of each other.

 

[roadstr6]

Check motor mounts as well. A friend recently brought his 60 over with a strange vibration. I noticed excessive up and down movement of the engine when I manually worked the throttle at the carb. I found bolts missing from the driver side front mount. A loose mount can cause all sorts of crazy noises and vibrations, especially when the exhaust bumps up against another part of the truck.

 

[1988hj61]

When I remove my foot from the accelerator, my '86, FJ-60 makes a noticeable and resonating vibration that shakes the whole truck. It generally occurs at decelaration from about 65-70mph or when I'm going down a hill, when I habitually feather the accelerator to regulate my speed regardless of the slope, and rattles the whole truck to the extent that I can feel it through the back of the driver's seat-- but no shimmy in the steering wheel. The truck has always had a certain looseness or slop in the driveline; if I'm not careful to feather the clutch release, the truck starts off in a herky-jerky motion. Yesterday, I was again nosing around under the truck attempting to discover the source of the vibration. When I grasped the rear drive shaft, there is noticeable free-play of about 5-10 degrees in the shaft rotation. Is this normal for this truck? Has anyone else encountered these symptoms?
Is this a matter of driveshaft balance?
Perhaps a mismatch between engine speed and rear wheel rotation speed?
The truck has 217K on it and has all original equipment.
Thank you in advance for your comments.

I am having this same issue, all these years later. Were you ever able to figure out the cause?

 

[Icantdrive55]

I am having this same issue, all these years later. Were you ever able to figure out the cause?

I never determined exactly what the issue was. After that series of posts, I had the engine rebuilt and new rear main seal, clutch plate and throw-out bearing installed. Could any of those have been the cause for the vibration? Having performed all those things virtually simultaneously, I would have no way to point to only one distinct cause.
Sorry I couldn't be more help on this one.

 

[CruiserTrash]

I never determined exactly what the issue was. After that series of posts, I had the engine rebuilt and new rear main seal, clutch plate and throw-out bearing installed. Could any of those have been the cause for the vibration? Having performed all those things virtually simultaneously, I would have no way to point to only one distinct cause.
Sorry I couldn't be more help on this one.

Could have been a clutch that was almost dead. Since the friction material is gone the pressure plate could be slipping/grabbing/slipping/grabbing/etc.

 

[1988hj61]

I never determined exactly what the issue was. After that series of posts, I had the engine rebuilt and new rear main seal, clutch plate and throw-out bearing installed. Could any of those have been the cause for the vibration? Having performed all those things virtually simultaneously, I would have no way to point to only one distinct cause.
Sorry I couldn't be more help on this one.

Sounds like it may be my version of a "death wobble" and just be a combination of ~40 year old parts getting a little too worn out. Thanks for the reply!

 

[1988hj61]

Could have been a clutch that was almost dead. Since the friction material is gone the pressure plate could be slipping/grabbing/slipping/grabbing/etc.

One day, I'll be able to convince my wife to let me change it, but for the time being, I still have the factory automatic. No clutch for me, but it actually drives really well, just a little extra sluggish.