I had driveshaft-related vibrations. They were rather speed-related than RPM-related. In your latter post you say it happens at 1500RPMs. Is this also true while standing and reving up in neutral? Is the noise rather rattling or humming?
Motor mounts, transmission mounts, cat heatshield, cat inside, differential mounts may be other components to be checked besides the ones that have been mentioned already.
Good luck, Bruno