CAR VIBRATES SERIOUSLY AT 40 KILOMETRES

My 2005, V8 5.7, starts vibrating as i approach 40 kilometers forcing me to slow down and
crawl! and when in traffic,it behave as if it wants to jump and it takes a lot of effort to hold it
down, i have changed all the plugs and i got a mechanic to look at it and he said the axle has
gone bad, but there is no noise coming from the axle. Another said the propeller is probably
bent but i have not had any major impact to warrant that. A mechanic brought down the auxiliary gear and said the problem could be from there.Please help before a major damage is
done to the car.