New drive shaft, new vibration at highway speeds

[e30jake]

I replaced my drive shaft with a refurbished unit, along with the CSB that came with it and a new guibo and transmission mounts. I also put a different differential in that didn't seem to have any issues in the car it came from. I did not replace the differential bushing. I drove it 200+ miles to Maryland and back on the day I got it back together. I had zero issues until last night, when I noticed a vibration through my seat at around 60 mph. It only happens under load. As soon as I let off the gas, it goes away. Taking it out of gear or putting the clutch in makes it go away too. If I set cruise at 60 (or above,) the vibration is there. I did make sure to preload the CSB. I did NOT adjust the length of the drive shaft because it seemed to fit fine. This is the only thing I am unsure about as far as the installation. I double checked almost everything else today under the car. I have seen several threads describing something like this, none of which ended with a definite solution. I'm pretty sure my rear subframe bushings are shot, which could be changing the angle a little bit under load and when I let off. That is really the only other lead I have at this point.

[e30jake]

Bump, getting under the car again tomorrow to re-torque/reinstall some things. Any ideas?

[SkiFree]

Check brand of CSP, have had batches of poor rubber in all brands (who are just repackaging the same thing) except Febi and OEM. Did you double check the u-joints on the "refurbished" shaft?

And.... shot it the dark ... Differential out of fluid?

[Killacortes]

Id check for play in the drive shaft. In one case one of my e30s had a bad output shaft connecting to the driveline

[e30jake]

No play in the drive shaft. I did check the u-joints on the new shaft as well, and they were nice and smooth. It seems I may have preloaded the CSB too much. I backed it off a little bit and it seemed to go away for the most part. There is still a slight vibration, but the window is smaller and it isn't as harsh. Updates to come.