Vibration from trans?

Hey all,

I have a vibration that sort of comes on and gets really bad at particular RPMs. It happens in neutral as well. Without a tach, I can't say quite what RPM, but it's easy to reproduce at 50MPH in 3rd gear. More gas makes it go away, less gas makes it go away.

I believe my motor mounts are totally shot, so I was planning on replacing those soon, and maybe even the trans rear mount.

Any thoughts on what to check?