What cause the vehicle to lunge forward or backward when in park?

THERE ARE ONLY THREE REASONS:

• It has a serious dangerous defect! Tow to be repaired, do not drive!

• It was not in park properly.

• The car was still moving when put in park

If the first is true it has a serious problem inside the transmission and must be repaired by a qualified person.

If the third is true you need to make sure the car is completely stopped every time! This abuse could easily cause the damage leading to the first example.