Sorry for the wording, they just call them Koni sport dampers in the video I was watching lol. I did try jamming on the brakes in a straight line today and that vibrating didn't show up but it still felt unstable when suddenly at a high speed and then going to a complete stop. It felt uneasy like the tires were just about ready to lose grip. So I think you may be right that it's the alignment, the shop that installed them said I won't need an alignment. I think he was full a crap.

an alignment is maybe a hundred bucks
most places will actually check your tires for balance and inspect your brakes for free, or a minimal cost

i'd do that before buying stuff

but eventually

coilovers and adjustable everything and sticky tires will solve alot

I have the sticky tires already, I'm going to check the alignment tomorrow at the shop

After an alignment and slightly softening up the front dampeners, that high speed skittishness went away. My Alignment when I went to the shop wasn't off that much but I tried driving and doing the same thing afterwards and it's definitely less nervous now. I was thinking it was a combo of things. Maybe the front dampers were too stiff up front and the alignment being a tiny bit off combined with me slamming on the brakes (all the weight going up front) then asking those same tires that are already at their breaking point to switch lanes quickly caused this?

Either way it feels a lot better. Learned my lesson, will be saving up for the Greddy KW Coilovers or the Bilstein B16 PSS9/10 if I'm impatient or wait to get the KW V3's.

Just my 2 cents, I had an 09 Sport Package before my current 14 Nismo, and it never felt unsteady at any speed under any circumstance. But after just a week of driving my Nismo I noticed exactly what you described, under hard braking it would quickly pull one way or the other and the back would feel loose. So I got rid of the 245s on the front and went to 275s and my problem pretty much diminished. 2 different solutions to the same problem? I don't know, but just a thought for anyone else reading this.

Interesting. That would make a lot of sense since the contact patch is larger. This may be a necessary upgrade for people going to the track or are going to be exploring the limits of their car? Even though I did the alignment and adjusted the dampers, I don't know if I'll see the same thing when I go to the track.

Doesn't take much of an alignment difference to totally change the feeling of a car.

On my R33 skyline I went from 0 rear toe to 1mm rear toe in and it totally killed the liveliness of the rear end, it was BORING to drive. On my R34 I told them to toe out 1mm at the back and it was beautiful (factory spec is 0 to Out 5.2mm!).

I have noticed some skittishness on the Z34, braking from highway speed. The car was almost new and maybe the brakes hadn't bedded in yet. Hasn't happened since but it's in the back of my mind.