Have you ever confirmed that the vibration frequency is a function of the driveshaft speed?  If we can free up an hour in Montrose I'll measure the frequency so you'll know exactly what the source of the vibration is.  If it is one times driveshaft RPM, its balance.  If its 2X driveshaft RPM, its angles.  You can't fix unbalance with angles and you can't cure angle problems with balancing.  

What's the origin of the shaft?

Have you tried the old hose clamp trick?

Out of curiosity, how do you know the vibration is coming from the driveshaft?

I bought it from Street or Track.  3-1/2” aluminum. 1350 ujoints

I sourced the aluminum driveshaft in my 68 from a 1998 2wd Explorer, had it shortened by Drivelines Northwest, and used a conversion u joint to hook it up to my 9" rear end. Maybe a new driveline is in order? Just a thought. I hope you can get some relief from your vibration issues. 
  
 

I had vibration issues with my 9” rear that I had assembled myself.  It was my first time performing this task and figured I did something wrong.  I had it rebuilt by a ‘seasoned’ person, who said the pinion bearings were bad (they were new when installed) and that I hadn’t done anything wrong.  (not sure he wasn’t just being nice or not)
While I still have some vibration, it has been significantly reduced since this rebuild. 

TKOPerformance wrote:

Maybe the preload wasn't set right.  Did you use a crush sleeve or a solid spacer?

It has a solid spacer.

I took the shaft to a driveshaft shop to see what it needs.  It will, at the least, get balanced, because there is no evidence it was balanced when i bought it new.  Maybe they got lucky and it just came out perfect, but I doubt it.

Will know in a couple of days.  In the meantime, I am going to replace the 160 thermostat with a 192, fail safe.

The shop said if the shaft didn't cure it, they could analyze the vibration if I bring in the entire car and diagnose the issue.  Probably should have done that 30 years ago, but most working parts have been replaced multiple times since then.

MS, does that fail safe thermostat lock in the “open position” on it’s first over temp?

Reason I ask, I removed what I believe is a fail safe thermostat from one of the 5.0L blocks and the only way for it to close and resume normal cycle was to manually reset it out of the car.

It locked the thermostat wide open.

The way I got it to close was to heat it up and before the wax cooled depress the tabs so it could close to normal closed position.

If manufacturers are doing this on purpose, you might be chasing an overheating issue caused by this type of thermostat.

Found a video explaining it:

https://motorad.com/products/thermostats/motorad-fail-safe-thermostats/?gad_source=5&gad_campaignid=21196815305&gclid=EAIaIQobChMI7sH26Z-3kwMVdFJ_AB3OERk0EAAYASAAEgIVs_D_BwE

“Once the Fail-Safe® thermostat does its job of allowing coolant to continue to flow to the engine during an overheating by locking in the open position, you must replace it with a new one.  There are standard thermostats, then there is the Motorad Fail-Safe® Thermostat. Ask for MotoRad®’s Fail-Safe® Thermostat the next time you need a new thermostat.”

Last edited by Nos681 (3/23/2026 6:19 PM)

We'll keep our fingers crossed. Either way, I'd be having a chat with Shaun at SoT, cuz those numbers are BS.

Last edited by rpm (3/25/2026 6:05 PM)

$228 later, my $500 drive shaft is straight and balanced. Now the vibration is 20% of what it was at 80 mph. Annoying, but not catastrophic like it was. 

I am replacing the thermostat because the car has a 160° in it and runs too cool.  If it fails, it sticks open, then you put in a new one.

MS wrote:

$228 later, my $500 drive shaft is straight and balanced. Now the vibration is 20% of what it was at 80 mph. Annoying, but not catastrophic like it was.

I am replacing the thermostat because the car has a 160° in it and runs too cool. If it fails, it sticks open, then you put in a new one.

Glad to hear the vibration problem has largely been resolved.  
 

I have tried all the suggested angles on engine and rear end. Next I will try what my logic says will fix this.  It follows comments from Steve Buzek about what a NASCAR guy told him. 

Engine needs to be closer to zero. 

While stock was 3°, they were not lowered like my car is.

By setting the engine at 1° or even at zero, it will raise the front of the driveshaft.  By setting the rear end down in front, it will lower the rear of the driveshaft.

My driveshaft sopes upward at the rear 1/2°. This makes front and rear working angles different.

Rough calculations show that, with engine at zero, the shaft will slope downward at 2° to the rear end at zero.
This should provide more favorable working angles of 2° on both ends.

Now, I just need more Round Tuits…

Last edited by MS (4/01/2026 9:50 PM)