Shakes violently at times - sometimes feels like there is something wedged against tires

For the past few wks, my Saturn Vue sporadically begins to shake violently while I'm driving. Once, it was vibrating and shaking so badly, that I pulled into the oil change place and they thought it was due to a caliper or brake line being stuck. I contacted my garage and they checked my Vue over and said they did not see a thing wrong with my Vue -they checked the brakes, rotors, and alignment. Monday, while driving home from work (4 miles), approx 1 mile from home my Vue began shaking so violently that I didn't think I was going to make it home. It almost felt like the tires were loose and going to fall off. It has also been feeling like it's "dragging" when I'm driving. Like something is wedged against the tires and not wanting to go when I press the gas. Anyway, Tuesday morning- car drove fine. All fluids have been checked (and are at appropriate levels) except transmission fluid because I can't find where that is. Any ideas about the shaking??