ok guys well i have always had this nasty vibration like a shake as soon as i hit 109mph. now its always dead on 109 never fails. now the other night i was able to get up to prob around 130 and it had only a slight vibration. now i never have poeople in m backseat and this time i has one kidk n the back and sum1 in the front.. now do u think it has somthign to do with holding weight over the suspension?? also wut could this be??

Pinion angle changes when you add weight to the rear.
I'm going to be trying this myself later on tonight to try and get rid of my vibration problem. The rear end may be pointing down too much (correct me if I'm wrong people).

When you lower the rear (i.e, add weight), the pinion angle changes and is probably coming into spec when the extra weight is added as the rear twists up.

Does anyone know what need to be adjusted in order to correct this? My car is at stock ride height and has a poly trans and torque arm mount. The torque arm is installed correctly. I wonder if we can shim something somehow in order to change the angle of the rear. An adjustable torq arm would do it too, but that's $$$.

What about engine mounts? Could saggy or old mounts cause a pinion angle problem? I bet mine are original.

ttt

The service manual says you add or remove shims from between the tranny mount and the tranny to adjust the pinion angle. The manual even gives the amount it will change and what longer bolts to use.
Of course this is the 1987 manual.

What kind of thinkness are we talking about?
I added 2 washers last night but it didn't help at all (my vibe starts at 75mph).

maybe pinion bearings are going bad or are bad. possibly low on gear oil will cause this. I changed the gears in my car and did not torque the pinion nut to spec(slightly loose) and it caused a vibration above 65mph. I tightened it up and it goes away...So based on this theory, I would say it was your pinion bearing going out.
Running that fast could cause heat problems in the rear end, which in turn may cause bearing problems.
Just making a suggestion

My Formula does it in drive at 58 mph. The motor mounts have been changed and so has the trans mount. OEM replacements? After hearing everyone else with the same problem. Your replys have me thinking it is a drive shaft problem. It's happening to us all at different speeds. Or is it a third gear issue? I don't think it has anything to do with people in the back seat. The cars were made with back seats for passengers.

I have a nasty vibration but it kicks is at exactly 110. 109 its smooth but 110 it sliek hell is under my car and it just gets worse and u go up

89, that's exactly my scenerio too - expect at 75. I don't know about down in the US, but here in Canada, It's dangerous to travel less than 80mph in the left most lane on the highway...you're sure to get rear-ended. Plus, it really sucks to have a car that pulls like a champ but I have to stop at 115km/h or so cause it will vibrate itself to death.

I've already blown a pinion seal over this and when it was changed, the pinion nut was torqed down properly. All bearings in the rear (including the pinion) were replaced last year....EXCEPT the wheel bearings.

I'm still leaning towards the shaft - even though I've had the Ujoint replaced and it professionally balanced..

what a pos

I will list the pinion angle adjustment from the Service Manual as soon as I get back to work after Labor Day Weekend.

Scott

Had a high speed vibration like what you discribe.

right about 100mph too.

Turned out to be caused by the factory aluminum wheels.

They were "balanced" too"
When I changed the wheels (same tires) it was gone.

Had nothing to do with the rear end, drive shaft or pinion angle.

Mine does the same thing except at 90........ I know it not my rear end cuz its new, never thought about pinion angle though, but when you put mine in netural the vibe goes away, I think my driveshaft is either bent or out of balance. But I will try adjusting my pinion angle, I wonder if mine could be caused by my engery supension tranny mount? Its quite a bit taller than a stock mount.

Tried installing some hose clamps on the front of the shaft yesterday and the vibration got a little worse. Moved them around and took it for another boot - seems to be 10% better now. I'm now convinced it's the shaft.

I've got one coming in the mail soon. I'll update once the new unit goes in.