1997 Suzuki Sidekick vibration

Hi, my sidekick vibrate and the more I speed up the more it vibrate, over 60 it is the worst, I tried changing and balancing the tire, and changing the driveshaft but it is not better. The vibration comme from under the seat and there is a sound and I can t tell what it is, the car go streat does not pull either way so my alignement must be correct. Where can I look next. Thank you very much. Jean-marc jacob