I have no idea but I have the SAME problem in my 2002 Dodge Grand Caravan Sport. I have taken it to three mechanics (Dodge included) and every time they look at it they say nothing is wrong, yet there IS something wrong!!! Mine also vibrates at higher speeds (Between about 70-100km/hr), the transmission has been checked and is fine. I wish I could get what ever is wrong fixed, it can't be good!!!
2) COULD BE YOUR DRIVE AXLE CV JOINTS