Pecuiliar vibration

[vex]

has cropped up in the last two days. I've been trying to isolate it, but it very hard to tell where the vibrations are based off of. I do not really feel it until I get above 40MPH and it seems to be oscillatory in nature. This leads me to believe that it's speed based and so a thrown weight, or something to do with the suspension and not the engine forward.

This seems to be confirmed when I free rev in neutral. I don't feel any increase in vibration similar to when I'm traveling at speed.

I am however still a little concerned with it as I don't feel any vibration under 40 at all. Vibrations also don't appear if I accelerate quickly to 40+. Which leads me to think that it's in the transmission/differential area.

Is there any other tests I can preform in the meantime to narrow the possibilities down?