Strong gas odor & shaking

I have a 98 Buick Park Avenue with about 115,000 miles and when we bought it three years ago, was in perfect condition. Well, for about the past two months or so, my husband and I have noticed a gas smell inside and outside of the car. Lately, the smell has gotten stronger. It seems worse when the heat is on and the car is idle, although at the same time, the smell is still pretty obvious when the car is running and the heat is not on.

Also, for about the past week, the entire car has began vibrating after hitting about 55 mph.

Any suggestions?