To follow up, can you put it in neutral and rev the engine up to 3k without vibration?

Since rolling at "vibrational speed" without the engine on produces no vibration, it seems unlikely that it's bearings or drive shafts. It's unlikely that it's the front end since you had the vibration with rear wheel drive and the front disengaged IIRC.

Is it RPM related? Is it at 3K regardless of which gear you are in? If it's RPM related, that would point heavily to pongoboy's suggestion of the flex plate or something similar.

Chip