Suspension issue need help

[Jason49311]

For the past week the whole car but mainly the rear end has felt extremely unstable, almost like it's floating at highway speeds. If I move the steering wheel from side to side at speed it feels like the rear end just wants to roll over or spin out. I'm currently running my winter tires, Pirelli Sottozero up front Blizzaks in the rear, 205/40/17 running 35-38 PSI all around. No major clunking and everything seems to be intact from what I can see. I live in St. Louis,MO. We have shitty roads and I dodge any and all potholes.

Mods:
Meister R coilovers installed about 25k-30k miles ago.
Whiteline adjustable sway bar links
TB Performance bracing front, mid, and rear
Rear sway bar
4 wheel alignment in January

Things I noticed beforehand. One of my front tires got a bulge in the sidewall so I removed both fronts and put the Pirelli's on in their place and drove around with 2 all season tires in the rear until I got the tire replaced last Friday. Upon getting the tire replaced I left the Pirelli's up front and put the Blizzaks on the rear. When doing so I noticed that apparently I forgot to tighten one of the rear shocks collars when I was adjusting the suspension prior to the alignment and the passenger side rear had moved about 1.5"-2" longer than driver's side. I put the rear shock back where it was supposed to be and zero change to the driving characteristics. Still sloppy as hell in the rear end.

I'm somewhat at a loss. The Internet says that it could be control arms/bushings, tie rod ends, rear axle beam bushings, or the shocks /struts. I paid $200 for the alignment at a reputable shop so I assume that if there was something wrong with control arms/bushings/tie rod ends he would have said something. Perhaps rear axle beam bushings, but the appear intact and as I said this happened all of a sudden or at least I really started noticing it when I had the tire issue. Could the uneven ride height shocks have prematurely ruined them? 95k miles on the odometer, no accidents. Any help would be greatly appreciated.

 

[M-Sport fan]

I am not claiming your problem is this with any certainty at all, but the Blizzaks are a VERY 'squirmy' winter tire when brand new due to their very soft outer tread compound, and extremely heavily siped tread blocks (what gives them their renowned ice traction), until you get down to the 'all season' type tread compound underneath (when @ 5/32nds I believe?).

But yes, I would double check the beam axle bushings as they could move a lot even before being so bad they would clunk, as well as all of the rear axle fasteners, including the upper and lower coil over mounting points.

I would also check the rear spring weight jackers/height adjusters to make sure that they are seated properly, and lock ringed tightly.

Lastly, I would try running your pressures at the factory recommended 3 PSI less in the rear than the front setting, regardless of what actual pressure you are running as a number.
(I purposely slightly over inflate my winter setup 40F/37R to cut back on some of the squirm, and help save the sharp edged outer tread blocks.)