I wish mine was that easy. It's only on decel between 2.5k-3.0k RPM, regardless of gear. Specifically, it's engine decel – this also happens while stationary.
It's more pronounced on the road because generally when I decel, I'll still be in gear and the RPMs come down much more gradually, so the vibrations stick around longer.
The vibration is deep, too. Like it's coming through the subframe or engine mounts and resonating the chassis itself. If it's vibrating and I give it just a hair of throttle and then lift, the vibe is usually gone. It's always gone as soon as I start accelerating again.
I'm hoping it's mounts, but I can't rule out the flywheel yet. But I just find it hard to believe that it's the flywheel since I have no other symptoms of a bad flywheel.