When its a U joint issue it will usually vibrate at specific RPM such as from 2,500-4,000 for example and anytime out of that range it will smooth out. So what I'm asking is does it do it at a specific RPM range? If so then its probably your U joints. Thats exaxctly the symptoms mine had when the U joints whent bad on my 95 GT. Was the perfect time to upgrade to an Aluminum DS