Was does my vehicle shimmy so bad?

Make sure your tires are balanced, lug nuts are tight, and brake pads are good.

it sounds like you might have a bad steering or suspension part. if the tires are out of balance you would get a shake evey time at certain speed if it was the brakes you would feel in when you it your brakes and with the with loose lug nuts you would feel it all the time. With the older cars we call this a death wobble...i would take it to your local shop and have them do a steering and suspension evaluation. it shouldn't cost to much and it will save you money in the long run