Okay so I have had a shimmy when braking and also a vibration when driving. When going about 75-80 the vibration gets weird for lack of a better word.
When braking it felt like your standard warped rotor. Not a big deal.
When driving, the car will have a slight rhythmic vibration that lasts for about 1 second on, and then 1 second off, and then returns. This will continue when holding speed or letting off the throttle and decelerating. I cant detect it under moderate to hard acceleration. It doesnt feel like a warped rotor or bad tire or balance issue. The frequency also doesnt increase with speed as when there is a rotational balance issue.
I never felt it in the steering wheel just in the seat, so usually a good indication its a rear brake/tire issue.
I took a look and confirmed my left rear caliper was seized. I replaced both rear rotors and pads, and the seized caliper (thank you Sewell). Bled the brakes and went off for a road test. The shimmy when braking is significantly better, but there is still a slight shimmy in the seat under hard braking. The vibration when driving still exists just the same. I had the tires rebalanced, but no improvement.
I rechecked the rear brakes and took them apart to make sure there wasnt some sort of rust ridge pushing a rotor off the hub. All looks good. No hot spots, no seized pads. I even checked the drive shaft to see if it tossed a weight.
Now im at the point where im concerned its a transmission or diff. issue.
Any thoughts? Im just stumped because it doesnt feel like your typical consistent vibration when driving on a bad tire or rotor.