Rygnar

Yesterday morning when I moved my RX330 it was making a loud grinding sound coming from somewhere under the vehicle near the rear. After driving down the street for about a block, a chunk of ice fell off the rear driveshaft. It was about 1" thick and it had formed completely around the driveshaft - meaning that it must have accumulated while I was driving the night before. I've never heard of this happening before. Have any of you heard of it or experienced it? Any theories about how so much ice could form around a moving driveshaft? Should I be concerned about it happening again?