It is definitely speed related. As it does happen in neutral when going down the highway. But I have noticed when I have the hood up it looks like the engine shudders to the same rhythm when I have it running in park.
And it's newish. Been going on for a year or so and I've had the car for 5 years. I'm going to try the diff angle plates, I'm not sure it'll help I just can't remember if it started before or after I replaced the leaf springs which definitely lifted the rear end up.