hmm i cant remember ever taking my truck up to 95....

 

They are S-rated, 112mph.
If I correct it, will I need more negative pinion angle or less negative pinion angle?

 

quaranteed you need your driveshaft highspeed balanced...
Gm balanced it for the governed speed and you are now going beyond that.
It is also common for the d/shaft to break at high speed on the highway as well as dyno's.
Stop speeding until you get it balanced.
do a search for "broken driveshaft" and see what you find.

 

How would I distinguish between an improperly balanced driveshaft and an incorrect pinion angle?

 

put your stock shackle back on and go for a drive, simple job

 

It did the same thing with the stock shackles... There was no change in intensity or at what speed it occurred. Come to think of it, since changing the pinion angle didn't change anything about the vibration, then logically it shouldn't be the pinion angle at all, right? I guess that just leaves the drive shaft...

 

Even though your tires were roadforced balanced and the tech said there good to go, rotate your tires to the front and see the problem goes away. If not then it is your drive shaft. I have seen tires roadforce OK but they still vibrated at higher speeds, they were just bad tires.

 

sounds logical to me , get it balanced-highspeed, before it breaks
here is a recent thread

https://www.performancetrucks.net/fo...d.php?t=435915

 

I will do that before I spend any more money. Although if I did that and it were the tires, the problem would just move to the front, not go away.

What RPM will I need to balance the drive shaft to if it is indeed the drive shaft causing the problem? I read somewhere that 10,000rpm is optimal, but nowhere I have called goes above 5000rpm.

 

I just took it on the highway again and noted the rpm that the vibration occurred at. In 4th gear with a 0.70 OD ratio, the vibration happens at about 2800rpm; in 3rd gear with a direct 1:1, it happens at about 4100rpm. Those two different drivetrain speeds produce the same driveshaft rpm of 4000-4100rpm...so a 5000rpm balance should be able to detect and fix my problem. I'll update once I bring it in.