Driveline vibration out of now where! Help!

Just thinking about your issue a little bit If it is a problem in 3rd gear, I would not think it is planetary related. Both front and rear clutch packs are engaged in 3rd and there is no gear reduction or internal movement in the planetary gear sets. I would verify the tail shaft bearing has not failed first. It I an easier repair and can be done in the vehicle. As mentioned I would pull the speed gear and take a look inside with a flashlight. Turn it by hand. If it is knocking as loud as you say it is, the problem should be obvious.