Q: Massive vibration after 40 mph or once I go from 4th to 5th gear.

Hi Darryl. Thanks for contacting us today. Generally speaking, a single cylinder misfire will gradually create a steady vibration, but not stop when you’re accelerating. The type of vibration you’re describing is often caused by a transmission or engine mount, a damaged input or output shaft, wheel bearing or inner engine component. The fact that it stops when you decelerate would lead me to start with the throttle body, and looking for either a vacuum leak or damage to fuel system components. To be honest, without being there to test drive or attempt to replicate the symptom, it’s really hard to ’guess’ - so you might want to have another mechanic come to your location to help you diagnose this issue.