Recently I had a Quaife helical diff mounted to my ST150.
I'm very happy with it. It really transforms the car's handling a lot.

I noticed a strange behaviour that perhaps wasn't there before so I would like to know your idea.
It's probably not caused by the diff but it just shows up now because I've been "testing" the car which is different than the normal "commuting" mode 🙂

When I shift gears, usually I gradually lift the throttle briefly before depressing the clutch pedal.
By doing so, the load change to the transmission components is minimal and the behaviour does not occur.
Buy if I depress the clutch pedal simultaneously with the throttle pedal lifting causing a more prompt load change, the front of the car shows a slight and brief jerky shift to the left.
It also happens when I engage the clutch in a less friendly way.
It doesn't do this when I suddenly brake, what makes me think it has nothing to do with the suspension parts (sheared or worn rubbers, loose bolts).
Something with the engine mounts perhaps or the subframe?
Any ideas are welcome.
Cheers.

Meanwhile I was suspecting the front control arm rear bushes.
I found Superpro has a "Steering-Pull Correction Kit".
link
This might even been better.
Is there anyone who has experience with these offset bushes?

Front control arm rear bushes have been replaced by Powerflex PFF19-1102BLK. This did not solve the problem.
It did give a better steering, without any adverse effect.

Then I replaced the lower gearbox mount by a Vibra-Technics FOR620M. This solved the problem for 90%.
It gives some more vibrations inside the cabin. Also the noise at highway speed seems to have increased.
But so far I can live with that. These inconvenients might perhaps get better over time. Some bushings tend to bed in after a while.

Hi. I know this is an old post, but if you still have the car, did the vibration settle down?
I too have a LSD and think my mount may need replacing as it gets a hard time from all the sealed motorkhana and autocross events I do.
Just wondering what mount is best without creating too much vibration.
Cheers, John
NZ