1996 Chevy Truck shaking problems

Shakes or Wobbles problem
1996 Chevy Truck 6 cyl Four Wheel Drive Automatic

Ill be driving down the road and my front end will begin to shake but it does so in a manner that it feels like the whole truck is bouncing and its hard to control while its doing it. It only stops when I come to a stop and sometimes it starts right back up when I get going again and sometimes it doesnt for a few days to a week. Could you please give me some kind of an answer on what is causing this.