Subscribed and giving this a bump as I am getting the same thing at the same speed range. Turns seem to amplify the vibration where turns to the right have a larger effect than turns to the left. Definitely somewhere in the driveline as it is not engine RPM related where it does not matter whether the clutch is engaged or disengaged with the engine at idle.

FWIW, dually rear drive.

Several fixes seemed to amplify the vibration a tiny bit, some did nothing, and only one helped.

And just to run down the questions of fixes (in approximate order) which did nothing to solve the vibration:
- New carrier bearing. Old one was very much gone. This was when the vibration appeared. Am suspecting the old blown carrier bearing allowed the driveshaft to absorb the harmonic.

- New single mass flywheel.

- New transmission fluid with some overfill.

- New shocks and added Sulastic shackles.

- New tires. Tire shop could have botched the balancing, but see the next item. New tires did help tamp down the vibration a bit.

- Installed Centramatic dynamic balancers. Do not recall whether this helped a tiny bit, or did nothing.

- New engine from Ford (Yes, New. Don't ask. At this point am accepting that the PO's did not take the best care of this truck despite its good looks). This had to get done and had nothing to do with the vibration. But am putting it out there anyway. No effect on the vibration.

- New U-joints and driveshafts balanced.

- Bearings in the rear axle replaced (some of them were reported as well past due) and new synthetic diff fluid.

- Transmission's tail shaft bearing reported as not healthy, but not a concern for reliability (yet) nor is it leaking.

The U-joints, driveshaft balancing, and rear bearings were done at the same time and seemed to amplify the vibration so that it is now greater than when I started on the parts throwing journey. My read is that I just removed a bunch of slop from the drivetrain and now the harmonic has fewer places to absorb.

Given all the above, which overlaps with pwjackson's experiences, the last remaining focus area is the transmission itself. Am checking with the shop to see if anybody in the area is able to suspend the truck while either letting the engine spin the powertrain or an external device spinning the wheels to better isolate the vibration's source. Am good with all the parts thrown at the truck to fix this vibration as they needed replacement anyway, but am to the point where I want to stop the guesswork. If the vibration does not appear with the wheels off the ground, it will shift focus to the suspension with a concentration on the front.

 

My truck has some vibrations and noise coming into the cab from the up and down pipes touching the firewall.

Sound comes and goes sometime while turning. I attribute that to clapped out motor mounts. I really should replace them or at least verify. But I'm getting ready to go on a trip and the trucks be sitting for most of the time.

 

This is definitely speed related. Harmonic hits right amplitude around 37 - 40 mph. With the recent round of new parts, seems like the whole cab resonates. Harmonic disappears by 42 mph. Noticed a secondary harmonic a little over 65.

 

Update. Got better information from the shop. They already had the truck strapped to a lift which left no doubt. 100% coming from the transmission.

Sorry pwjackson, but, now we know.

Maybe it is time for a ZF-6 conversion

 

Check both yokes. trans and rear end.. My 1995 zf5-42 had a worn yoke in rear of trans, but you could not tell unless you removed u-joint and inspected. Look very carefully for smoothed out shiny areas. Spicer quit making them though, and if you find one it will be up around $500. Also it may be worn when you get it. I ended up doing a zf5-47 swap just because at that time it was easy to find parts for it and it is a better tranny. Good luck

 

i had a strange vibe on my 88 4x4 at 40 mph to 70
dealer rebuilt the trans twice under warranty but it came back after a few 1000 miles

then ignored it until 100k and the pinion bearing let go
new to me axle installed and that noise was gone until i traded it in at 180k miles

 

ok thanks for info