Does this only happen when you break? if yes then it could be your rotors or even brake pads. Check these two first, if they are fine, then check you ball joint..bushings...etc.
If it shakes even when you are not breaking just driving along, then it could be your tires or rims. If they are fine then for sure get your suspension components checked! Hope this helps....
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Body: 318i 4 door
Mods: M50 engine, Jim C chip, light-weight flywheel, dual cat-back custom exhaust, 3:15 lsd, euro headlights w/angel eyes, 5300K HID, smoked corner lights, FK Motorsports foglights (crystal clear), ABS plastic upper eyebrow, clear side markers, M3 powered side mirrors w/LED, blacked out grill, M3 lighted shift knob, 17" ROH Furry wheels w/Kumho tires, Brembo cross drilled rotors and Hawk performance brake pads all around, PSS9 Bilstein and H&R coil over kit, front strut bar and Alpine alarm.