I have a 2014 Mustang V6 with a 6 speed manual.
When I drive at any speed, and let off the accelerator, there are times the car will vibrate like it's driving on a rumble strip.
This happens between 30-40 mph in 3rd gear, and 60-70mph in 5th or 6th. What's real confusing is that I can't seem to replicate it on a consistent basis, and it goes away as soon as I start to accelerate again, or depress the clutch.

I did lift the car and checked the driveshaft (2 piece drive shaft), and everything...

Possible Driveshaft issue