1. A big rock fell off a truck.

2. A big rock fell from a truck.

(Are both the same?)

3. The rock fell onto the street.
4. The rock fell on the street.
5. The rock fell to the street.

(What is the difference among the three? Are they interchangable? Are they all correct? Which one is suitable?)

1 and 2 mean the same, yes.

3, 4, and 5 mean about the same thing. They're pretty much interchangeable. The best one is #3.