These have been my observations so far on the shudder that my car exibits:
It doesn't happen all the time.
It mainly happens at idle or low RPMs, and moreso under load (such as pulling into my sloped driveway). It doesn't matter if I'm going forward or in reverse. It seems as if the lower the RPMs, or the slower that I go, the worse it is.
If I'm sitting stopped I've noticed that if I push in the clutch, the noise winds down and goes away (as if it has something to do with the rotating mass of the transmission input shaft). If I release the clutch it comes back.
Sometimes it can bee quite loud, and the car will shudder and buck quite violently when taking off from a stop.
The fact that the noise goes away when the clutch is pressed all the way in would point toward the transmission (input shaft bearing maybe?), BUT it could also just be the unloading of the throwout bearing, pressure plate, etc.