I have had this issue since i got my 9 inch, but never got around to fixing it until now. There is a damn vibration that rattles the whole car once you get over 80mph. when you get to around 120mph it is so bad that it shakes everything in the car and my rear view mirror fell off. It ONLY vibrates when you let off the gas. If you are even slightly on it, there is no vibration at all. what gives?

I have a PST 3.5" alumnimum DS. did i buy the wrong one? i got it from EPP. Thanks for any suggestions

 

Could be bad wheel bearings, since you got the 9inch and that being last thing you did before noticing it, does it make noise?
Have you got a chance to see if its leaking?

 

This may seems odd but if its shaking at 80 why take it to 120. Are you getting any noise, does anything look oddly worn?

 

know this is a very uneducated guess but sounds like a drive shaft

 

dbl check your pinion angle.

 

Find a local driveshaft shop, have them verify the balance. Then go from there.

 

this is also what i was going to suggest, sounds very logical considering its no longer a stock rear end

 

Sounds like a loose lower control arm. I used to have the same issue and thought it was the DS. Switched it out, with the old stock one, and it still did it. Checked a few things, and found a loose LCA bolt. Tightened it up, and the problem went away. It'd only happen at 70+, whenever I let off the gas.

 

x2 I had a loose LCA once too w/ similar symptoms, its also the easiest thing to check so start there. next check to make sure all your tq arm bolts are tight and check the pinion angle like stated(although I think if the angle is off you'd be feeling it constantly?). also check your tranny mount. if still no luck I would probably get a high speed balance of the tires next, especially if they used the magnetic weights that sit inside the rim. although it would probably be cheaper to check the DS balance first so you might want to do that before the tires. good luck & keep us posted!

 

driveshaft is too short I guarantee it 100%. I had the EXACT same problem with my pst shaft, they sent me a stock length, and a 9" needs a 1" longer shaft. get under the car and see how far out the yoke is from the output of the transmission, anything more than 1/2" and you'll get vibration.

 

I have a PST aluminum, correct length, DS. It vibrated horribly at 80MPH. FTR it had no balancing weights on it since it came out of the box.

I called PST and they askem me to send it to them for a checkup. It returned with a pair of balancing weights on both ends of the tube and the nasty vibe is now 95% gone. I think I may have to replace my tailshaft bushing again becuase I think the unbalanced DS might have worn it excessfully.

Do not balance it locally. Call PST, send it to them and let them check it out. They were excellent to work with and they will stand behind the product.

 

Any link or more info that confirm this.Dont worry its not because i dont believe you but i got a vibe just a little over 70mph.Diff has been check 3 times,pinion angle is good(tested 0 degree -1 and -2 and still vibe).

So before changing my DS(yoke is about 1 inch out) i would like to be sure

 

nope, just personal experience I didn't believe it either, until I took the shaft to check balance and it checked out perfect. The driveshaft shop saw the wear mark on the yoke from the rear seal on the trans, and assured me that was the cause of my vibration. Called PST and they sent me a new shaft, fixed the problem.

 

I have the same problem with mine, it started vibrating at 120 then when I changed my gears it starts at 75. Found out my driveshaft was twisted a quarter turn, got that fixed and I still have the vibration??? Anymore ideas

 

Most of the time vibrations 70+ are usually caused by the drive shaft. Worst case senario, find a place with a good return policy grab a new DS then install and take it over 70 see what happens if it still vibrates send it back.
Unless there is a way to test or get your stock DS balanced for cheap...

 

Check cheapest things first

wheel balance, check a weight hasnt spun off rim,
engine mounts
arms
drive line

 

Chances are if it was the pinion angle you would have the vibe a whole lot earlier than 80mph. More like 30-40 or so, at least in my limited experience anyway. It could be other things but I'd put money on a drive shaft issue. Either the balancing or being too short like wicked said.

 

There is no right or wrong way when it comes to mounting a driveshaft. It is not balanced based on what position it goes onto the yoke.