I have a 2008 Civic EX/5spd with 83,000 miles. I have mild-to-moderate chassis vibration starting around 45mph under power in 3d or 4th as I accelerate, but it's less noticeable in 5th at highway speeds (e.g., 70 mph). Vibration ceases if I'm in neutral at the same speeds.

If it was present both under power and coasting, I'd say it's a wheel balance issue. Not the case here. I have a new left half-shaft (CV joint) but the right one is original. My best guess is the Right CV joint has some play and needs replacing. Any thoughts?

 

Inner CV joint. To confirm, jack up front until tire is off the ground.One tire at a time. Put in gear and simulate speeds where the vibration occurs. Have someone step on the gas while you peek under the car. Watch for excessive movement of parts.

 

Or get under the car, both wheels off the ground, engine not running, and check the joints for play by trying to move them by hand.

Letting one wheel spin at highway speed with the other one stopped has been known to blow the differential on some cars.

 

You were right; it was the inner CV joint (right) that caused the vibrations. R&R CV joint and it has resolved the issue. Thanks for your input.