The prop shaft bearing is the driveshaft bearing and is what I mentioned earlier on in this thread. It is common for it to fail. Not always necessarily the BEARING itself, but the whole support assembly. That's why I was curious about what you had felt.
Because you can feel it and based on what you described, it seems like it isn't perfectly centered anymore and the support has gone soft/weak. Certain speeds and or load can manipulate the driveshaft's position. If this is in-fact the cause, pro-longed driving in this state can damage the flexible joints on the shaft.