BSRJIRONMAN

Hey guys, figured this would technically be in the s-chassis section, here's a little background,

I've got a Z31 that I've swapped an S13 multi-link into. I've got a severe vibration coming from the rear suspension from 45-65mph. It feels nearly identical to driving on the rumble strip. Here are a list of modifications in the drive train:

-Stance solid subframe bushings (press-in).
-Completely new balanced driveshaft, including u-joints
-welded diff
-ichiba rear 5-lug hubs
-ichiba 50mm extended studs
-d2 coil-overs
-50mm wheel adapters
-progress adjustable sway bar w/solid links
-no name RUCAS
-new rotors/pads

The vibration was not present pre-swap, so it's got to be something in the rear end. I feel it through my seat, not my steering wheel. At 60 mph, it feels like a massage chair, and my teeth feel like they're going to rattle out of my head. When assembling the rear components, I put thread-locker on all hardware before torquing down.

I've never driven in a car with solid mounts and welded diff etc., but the diff is making all kinds of noise. It clunks if I shift aggressively, and whines a lot. Also of note, when cruising at vibration speeds, if I accelerate hard, the vibration completely disappears. Then when I let off the throttle, it's right back in massage mode. The vibration tapers off after about 65mph. Another thing; when I was installing my drive shaft, there was about .625" of "free movement". I was told this is normal, but I just thought I would mention it.

My guess is the diff is the culprit, but I'm interested in hearing your experiences and solutions. Thanks alot!