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So, I'm at the point of being pretty pissed.
Installed a used 8.8. Problem when swapping gears was the 8.8 had been sold as ready to go with a little gear whine.
I had new carrier installed because the one in it was junk.
Swapped bearings because they were junk.
Installed it in the car, vibration was so bad at 70 it would be scary to even think of going faster.
Dropped the drive shaft off to have it checked(came with the rear end) and when I picked it up yesterday, I was told the trans side that was welded in was crooked/off center. I was excited thinking that the problem was going to go away.
Install the DS and test drive, good news is that the vibration is not so violent and bad news is it is still there.
So, fresh rebuilt diff, fresh u joints, new yoke and fresh balance. I'm out of ideas.
Has adjustable TQ arm and I'm running -2.5 pinion angle like I do on all my f cars.
I'm going to check the trans yoke bushing today again, otherwise I'll throw the 8.8 flange on the stock aluminum DS and try it again.
Open to any suggestions because I'm running out of idea.
I doubt many of us could tell you something you don't already know. I can tell you I have to run my pinion at -0.5 degrees to keep the vibes down. Is it possible the driveshaft is too short?
Your torque arm hook to the tranny or a chassis mount? I remember one guy posted his was vibrating and it turned out his torque arm was not pointing straight forward, which pushed his pinion out of alignment horizontally.
Last thought - if the vibes are not audible at 70mph and you can feel the individual pulses, it could be something on the axle / wheel side and not the driveshaft side. When my driveline vibrations were there, there was a low hum coming through the floor at highway speed.
The drive shaft is not in as far as it should be but it is the same as my 89 RS was when I swapped the 6.0 T56 to it. Maybe 3/4 of a inch out from perfect.
The TQ arm is mated to the trans crossmember. Welded that up myself. Same spohn TQ arm I had in my 89 Camaro.
Said F it today and didn't mess with it since my drive shaft guy was out of the shop early so didn't have time to take both the stock DS and the one that came with the rear end to see what U joint I needed to swap the flange to the stocker and try it.
At neg 2.5 pinion I'll start increasing and then decreasing to see what happens.
Some said they had vibration till they went to 4.5 neg pinion.
There are so many possibilities and that why I posted so see if there was a chance someone would come up with something I have not researched or thought of myself. I never heard of a TQ arm angle causing a issue. Since mine is adjustable I doubt it but since you mention it I will mess with it if I get that far and don't find a different resolution.
Good luck, man. i'd rather change a clutch than chase vibrations down. Took me months to fix it. Please keep posting back if you find anything.
Random thought, but increasing your pinion angle might get you into the "broke-back" configuration. Here's a pic of my angles where I ended up compared to the broke-back. Not telling you something you don't know, but throwing it out there:
This is an attempt to depict the angles that worked for me. The angles are all relative to level with the earth.
I've seen this work on cars before also. Maybe this is why the car liked increased pinion angle. In this arrangement, you want both interior angles of the U-joints to match.
I was going to lose it. But, I threw the 8.8 flange that bolts to the rear diff on the stock drive shaft and it's perfect. Makes no sense since the damn steel shaft was new everything and balanced but whatever. Just hope I don't break it playing at the track.
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Vibration issues 
