okay i have a 99 3.8l v6 5 speed mustang. when i start the car it vibrates out of the normal and when goin through the gears the shaking gets unbearable. i bought the car used guy said he had a mechanic put a different engine in it because the old one had a burnt valve claims to have had it balanced also. now i thought the motor mounts were bad and as i was gettin ready to change them a friend said to check the transmission bolts. come to find out there was 2 missin out of my transmission could this cause the spacer to be worn out or to have chewed up my flywheel? also car looks like it has been rear ended could this have bent the drive shaft or done something to the rear end?? also has had clutch replaced. what do i need to try?? iv replaced plugs plug wires rotated tires any help is much appreciated