I have a "well used" 182K 3.1 Lumina sedan with a front end vibration. The vibration stops during acceleration or braking and is speed-affected: it increases frequency as speed increases. I've just replaced rotors and pads because I had a warped rotor and the braking is smooth now though it was quite rough before the repairs. I just noticed the neutral-throttle vibration this morning on my way to work. I will be putting new tires on the front tomorrow but, when I inspected them when I got to work, they did not appear to have any visible vibration-inducing defects such as flat spots, exposed cord, bulges, or splits.
Thanks in advance.

This can be tough to isolate, if the tires turn out not to be the issue, I'd be looking at a possible ft axle issue, the inner cv joints in particular can cause this.

Yeah, a nuetral load vibration or shake is usually gonna be a an axle joint or a motor/tranny mount, both of which are free to shake when there's no torque being applied...

my 91 Grand PRix had a vibration like yours and it ended up being a bad outer tie rod. It would start at about 40-45, and progressively get worse. When you let off the gas (or hit the brakes) and got back on it, it would stop vibrating, then start again.