Weird vibration when I decelerate

When I remove my foot from the accelerator, my '86 Land Cruiser makes a noticeable and resonating vibration. It generally occurs at decelaration from about 60-65mph 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 I can feel it through the back of the driver’s seat. 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 nosing around under the truck attempting to discover the source of the vibration. When I grasped the rear drive shaft (it has a second drive shaft for the front wheels), 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? Thank you in advance for your comments.

Sounds as if your U-joints are shot and need replacing.

Noticable free play of 5-10 degrees is NOT normal. Check the amount of movement when you rotate the driveshaft versus movement at the transfer case yoke and rear axle yoke. The universal joints should be much tighter than that, and I think yours are shot.

I dont believe any free play should be there. My terminology might be wrong, but I think you need a new driveshaft spline or yolk. I dont think its uncommon for this repair to be needed on older vehicles. It might be a bolt up item or it might have to be welded on and then rebalanced. But I think you’re in the right direction with your hunt.

Im guessing that if you’re decelerating from 60-65 and you push in the clutch to basically kill the RPMs then the vibration goes away?

One other thought though, or question; If the driveshaft was the culprit, my guess is that whatever RPM youre at during 60-65 mph, the problem would present itself at other speeds doing the same RPM. So if say you’re in 5th gear and 2700 RPM and the vibration occurs, I would guess that when you hit 2700 RPM while decelerating in 4th gear doing 45-50 then the vibration would happen as well.

Careful inspection of the rear driveshaft by an experienced mechanic should be step one. Some subtleties can be lost on you if you don’t have experience.

I once had a case where the driveshaft was slightly bent from hitting a hoist and it was never diagnosed until it came to my shop.

That amount of play isn’t normal. On my Ford 4WD truck there is absolutely no free play at all when doing the experiment you did. I think in certain vehicles there could be some play observed by twisting the driveshaft, but that play would be coming from the differential and xfer case. If you hold the yoke (on the diff) still so it can’t move, then there should be no angular play at all at any point in the driveshaft. If there’s the amount you say occurring in the driveshaft, you should be able to easily determine between which interfaces it is occurring.

And there’s your vibration when slowing.
The slack, whether in ujoints or ring & pinion or both, allows for the disagreement between the engine’s push or rest and the rolling slack of the gears or joints.
( a liitle hard to describe, but…)
As you accelerate there’s constant push against the compononts.
When you decelerate the engine is not providing that push yet the wheels are still rolling through that space in timing to catch up to the engine’s rpm then the back and forth catch up / slow down happens multiple times each second creating the vibration.

to test ;
Put it in neutral to see if the vibration goes away or add just a tad of accelerator ot play catch up with the gears.

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I’m having the same issue with my truck, on mine vibration goes away when I put it on neutral…
any clues what it might be causing the vibration?