i just got my car back from bmw today (they had to fix a brake problem---noise coming from parkin brakes)
but yea...when i got on highway the car was vibrating so much i could feel it on my butt like a massage...i got scared

 

You should have turned around and gone straight back to the dealer. It could be any number of things. Unfortunately, hopefully, its the obvious (a rear wheel not put back on properly). Especially if they are after market, a centering hub ring may be miss-alligned. Another possibility is they knocked off a wheel weight and its out of balance.

 

Sounds like wheel balance ... particularly if the problem is over a particular speed ...

 

+1 wheel alignment!!! re-post by the way!

 

+2... I'd have a wheel balance done first.