Prop shaft vibration
 

In preparation for a future engine swap I wanted to replace the 12 rusted and rounded allen bolts securing the prop shaft with new OEM bolts and washers. Quite a pita had to cut off most of the bolt heads to get them out.
With the prop shaft separated from the car, I figured why not inspect and service it before bolting it back in. Popped of the end caps, cleaned most of the old grease out, and packed it full with new grease (mos2). Everything seemed to look good.
I bolted the prop shaft back in the car, front to front and rear to rear (flat end cap to diff, end cap with cutouts to the trans), and torqued all bolts to spec.
Regarding orientation/clocking of the prop shaft, I did not bother marking it on removal. I read on here that orientation doesn't matter, even saw billman say this twice in two separate threads.
On install I did try to clock it correctly using the paint marks on the prop shaft and trans/diff flanges because why not, but not sure if I did it correctly due to faded paint marks.

Went for a test drive, and now above speeds of 60 mph I am getting a very heavy vibration. It is mostly road speed dependent, accel/decel/steady doesn't really matter, sometimes it's worse on slight decel, sometimes keeping the throttle steady is worst.
It's quite a violent vibration and can be felt through the entire car; steering wheel, pedals, seat, everything. Also there is very low pitched sort-of humming sound.

Not sure where to go from here, what can I do or check next to try and fix this problem?

Did you double check the bolts and nuts? After the run?

Yes I did after the test drive, they were all still tight. Also while the car was up on jack stands again I ran it to 60 mph with the wheels in the air, but it was very difficult to pinpoint a source of the vibrations.

You ran your car to equivalent of 60MPH on jack stands? Makes my hands sweaty just thinking about it. I hope you weren't under the car, at the time.

https://cimg9.ibsrv.net/gimg/www.s2k...adb9ea77f1.jpg

A car with driveline vibes at that! :-O

To repack the cv's, you removed them from shaft, correct? Are you sure it all went back together the same regarding front vs rear? A lot of the cv pieces are same front vs rear, so that would seem easy to do...

I have jack stands with very deep notches, the car isn't going to fall off :) But that isn't what this thread is about.

I fully serviced one cv first, and after that the other. So mixing up parts front to back was impossible. Unfortunately this cannot be it.

The CV cups develop a wear pattern after a while and will eventually vibrate, typically under heavier throttle application first, its a known fix to swap the cups from side to side to change to a new wear position. It doesn't explain why you have vibration now, but its something Id likely do in your case to rule out and may likely solve your vibration issue. Otherwise, sounds like a wheel balance issue.

isn't that mostly true for lowered cars? I thought stock height wasn't really affected. I'm dealing with some vibrations too at higher speeds.

No, that's just what people seem to have adopted as the cause or being sold through the aftermarket, when in reality its normal wear and tear and SOMETIMES when you lower the car, it exacerbates an already wearing cup, when in reality lowering isn't the cause. All CV/cups wear and eventually need to be addressed. You can address them two different ways, and both ways should be done at some point if you want to get the most mileage out of them. Swap the cups from side to side is one option the other is add spacers, both change the old wear path. Do one of those options first, then when the vibration comes back again, do the other. I got 220k hard miles out of a set doing this before I had to buy new cups.

Is the vibration an issue only under load? If so I agree swapping cups is a good idea. It's good practice in any event I think to balance the wear that occurs.