[maxwedge]

If you shut the engine off and coast down the hill and it vibrates I would look at inner cv joints or possibly the tires need a road force variation balance done, but that would be 2 sets of tires doing it, unusual. GMmerlin's advice is on target, but most dealerships don't want to spend the time/money to perform these tests. Careful if you try what I suggested. No brakes and steering shutting the engine off!!