Vibration when braking need some help!

1cleans13

Hey guys, I recently (5 days ago) lowered my car on Swift springs and installed H & R 15mm spacers... I havent messed with the alignment yet. I noticed yesterday while driving the car on the highway at about 90 mph I pressed the brakes to slow down and my steering wheel started shaking. I checked the lug nuts when I got home and at least by hand they feel tight enough. However I noticed the 2 rear wheel locks are loose and broken (meaning I could see the stud)
Any Idea as to what the hell is happening here... My first thought is someone tried to steal the wheels but my car sleeps in my garage and I dont take it anywhere but to work on a secure lot.

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2011 NISMO 370Z (PW) 50% Llumar tint- LED bulbs everywhere- JDM emblem- Phillips LED DRL- 6K HID- NISMO TI shift knob - SWIFT SPEC R - H&R spacers