I have got a set of a mint 18" amg monoblock et31 18x8 all around for my w202 c280, fitted with brand new 225/40/18 firestone wideoval tires and got them aligned ... after they were fitted I have noticed that my car started to shake & vibrate at about 55MPH and vibrates more once I go faster, I feel uncomfortable with this shaking,and BTW the car have a completely stock suspension and the shocks have been replaced a month ago if that helps.
any help will be appreciated
thanks in advance.

did you use the correct wheel bolts? get the wheel/tires rebalanced.

you might need spacers

This comment is made assuming your rims are true and not bent. There are minor imperfections in the wheel and tire when they are manufactured. When you purchase a new car, you will notice a little dot or paint spot on the tire and wheel which indicates the low spot of the tire and the wheel. These two are lined up to achieve a more optimum balance. On used rims and most aftermarket tires are not marked, creating minor balance issues. To address issues like this, Jaguar and BMW require their dealers to balance wheels using a Hunter Road Force balancer. This balancer actually applies force against the tread to simulate the forces experienced when the car is on the road. This system is very expensive, however there is a good chance it will solve your issue. So, find a dealer or tire place with a Hunter Road Force machine to balance your wheels. Places with this kind of equipment generally have more expertise in these kind of problems.

all is true... you really start feeling the vibrations or shake after 45mph... perhaps one of your front wheels needs rebalancing...