Do you have a dodge? Your Profile says you have a 1983 GMC Jimmy? If you have a dodge, From the info you gave it sounds like its Death wobble. Search the forums here for "death wobble". http://www.pavementsucks.com/forums/search.php Im not sure if GMC Trucks have the same problem. There are a lot of postings on it. When was the last time it was aligned? Its most likely your Track Bar. They wear out fast. You might want to think about getting a after market one like Thuren ( http://thurenfabrication.com/trackbar.html ), DT or something similar. Most here had the same thing, Including me.