I was driving around before, and i noticed that when the car downshifted (its an auto) to i believe 2nd gear, it hesitated, then shifted and bucked hard, then the car began vibrating pretty heavily while i was in that gear. my mechanic seems to think i need to change my transmission fluid (i explained it to him over the phone). Can it be as simple as that or do you think my trans is going. Any opinion would be great