Shakes or Wobbles problem
1994 Buick Lesabre Automatic 128K miles

My car started jostling back and forth when I drove slowly, in traffic, about a month ago. It literally rocked me side to side, like riding a horse. At that point it was only doing that at 20mph or under. Then it moved up to 30mph. Now it shakes - like a high speed shake, which i only ever had at 75mph - whether i'm doing 55 or 65 and still jostles horribly at slow speeds. There are no accompanying sounds or indications of a problem. What's wrong and is it safe to drive like this for another week or two? Thanks!