We do that in some of the tractors for more weight.

 

I agree on that, it could be a bad U-joint or a loose U-joint, just check the drive shaft joints...
or you can test it parked, engine running, with brake pedal pressed all the way, changing gears from reverse to drive, and from drive to reverse, passing the neutral very quikly, do it for a few times, just don't let the vehicle move, If you hear a solid metal noise, you have a bad or loose U-joint an that can cause vibration at high speed. Hope it helps...

 

I don't mean fill the tire with liquid, just 8 liquid ozs, which comes out to be 12 ozs of weight (I weighed it).

Do some googling. It's a common practice in heavier tires. They make Dyna-Beads (a special sand) that does the same thing. The science of why it works is complicated, but it does work.

 

I know you didn't mean fill it, I was just saying that I've heard of a similar practice.

 

I have tested the U joints and not found them loose or noisy. The rod test as you suggested has been done and no the pattern does not change.

 

Dyna Beads....interesting.

http://www.innovativebalancing.com/index.php

 

Some use Dyna Beads in their motorcycle tires.

SJ02

 

I'm going to disagree with that only because using this technique could be incorrect if the differential (ring and pinion) backlash is out of specification is out or worn it would give you the same "Metal noise" sound. Most older vehicles gear sets are worn.

selkov<--- Just because you don't feel anything doesn't mean you have one that's bad. You have to first mark the shafts to there respective placement on the yolks then remove the saft and physically feel each bearing to determine the possible issue. Also check for any missing weights.

The front shaft could also be the issue as well as the CV (half shafts) joints.

Finally what about tire pressure? I used to run 38 psi in the front and 36 psi in the rear if I remember correctly.