Vibrations at certain speed under load?

[FatlessLemon]

Hello everyone! I've got an 86 HJ60 that has an annoying vibration.

Whenever I get up to 60kmph (on the speedometer with 33s) I get a pretty noticeable vibration. It feels like the torque converter locks up right at 60 and then I feel the vibes. It almost sounds like a whir whir whir sound. When I get up to speed around 75 to 80 the sound and vibrations dissappear completely. And if I shift to 3rd the vibration also goes away. Its seems like it only comes on under load as well because if I left off the gas it goes away.

I previously thought it was the torque converter because i used to get a bad shudder around lockup speed, so I replaced it with another used transmission that was apparently rebuilt recently but no change the vibration is still there but the shudder disappeared.

Ive also swapped both front and rear diffs to see if there would be a change but again no difference.

Ive also replaced the ujoints in the driveshaft with no difference except maybe I dont feel it as much in the driver's seat? Its hard to say though.

Anyone have any ideas of where I should look next?

 

[fjc-man]

Has it been lifted? Maybe the rear driveshaft angles are off calling for shimming the rear springs.

 

[FatlessLemon]

Has it been lifted? Maybe the rear driveshaft angles are off calling for shimming the rear springs.

I was thinking that too. It has an old tired OME lift on it. Theyre starting to flatten out a bit it looks like. The angles on the flanges on the transfer case and differential seem to be correct.

I had a running theory in my mind that the leafs and bushings are worn to the point that I get a bit of axle wrap when the torque converter locks up thus changing the pinion angle enough to cause a vibration? Not sure if thats a possibility.

 

[ceylonfj40nut]

Share pics of truck from side to look at “lift”, share under carriage pics of rear drive shaft with connection to transfer case and rear differential.

Obvious Qs
1) if you get under truck and try rotate drive shaft any play?
2) unjoints and slip joints greased?
3) does whir whir frequency change with speed?
4) is rear drive shaft in phase?

 

[FatlessLemon]

Share pics of truck from side to look at “lift”, share under carriage pics of rear drive shaft with connection to transfer case and rear differential.

Obvious Qs
1) if you get under truck and try rotate drive shaft any play?
2) unjoints and slip joints greased?
3) does whir whir frequency change with speed?
4) is rear drive shaft in phase?

 

[FatlessLemon]

Share pics of truck from side to look at “lift”, share under carriage pics of rear drive shaft with connection to transfer case and rear differential.

Obvious Qs
1) if you get under truck and try rotate drive shaft any play?
2) unjoints and slip joints greased?
3) does whir whir frequency change with speed?
4) is rear drive shaft in phase?

1. No play in driveshaft
2. Ujoints recently changed and greased and slip joint greased.
3. Doesn't seem to be a change in frequency.
4. Driveshaft is in phase.

 

[toy_tek]

I was thinking that too. It has an old tired OME lift on it. Theyre starting to flatten out a bit it looks like. The angles on the flanges on the transfer case and differential seem to be correct.

I had a running theory in my mind that the leafs and bushings are worn to the point that I get a bit of axle wrap when the torque converter locks up thus changing the pinion angle enough to cause a vibration? Not sure if thats a possibility.

Not a terrible theory... and consider your engine & trans mounts too. If they're worn out, your output pinion angle could be affected. Trans mount could slump down over time. Rather than guessing, remove your rear driveshaft and use a level to measure and compare the flange angles - there are free phone apps that will get you in the ballpark well enough to see if there's an actual issue or not. Xfer case & rear pinion should be parallel to each other (or as close as possible).