How is your mount system set up, Dave? If there's not a mount at the tailshaft section of the trans, there might be enough torque on the transmission case to make it shake side to side. If that's the case and the transmission is only supported at the bellhousing, try attaching a couple of nylon ratchet straps around the tailshaft. Loop them over the frame rails at each side, and put some tension on both straps with the ratchet. Take a test drive. If the chatter goes away, or it's reduced noticeably, you might want to consider fabricating a rear crossmember with a rubber mount on it.
Jerry
