it won't drive forward more then a few feet.

My car was just at the dealers for a tune up, nothing wrong. Well, the tow truck just picked it up because now it wont go forward more then two feet then the back wheels lock up like someone just hit the breaks. But wait it gets better, when the tow truck driver was getting the vin number I found two screws under the area where the wipers are when the river popped the hood turns out they belong on the engine. And it looks like even through I only found two there are four missing. I was wonder what are some of the things that could cause a rear-wheel to not go forward, oh the breaks are 3yrs. old and I don't know if they checked them. HELP!!!