Same here. In fact if someone really wanted to get technical with it you could use hybrid bearings in places like the cups on the diff. You could run a bearing thats rubber on one side and face it toward the outside of the diff cup and metal on the other side that would face inside the diff cup...
WF is right if theyre in a sealed environment you technically dont need shields but if you're going to use bearings with shields the metal shielded would be better. Metal shielded bearing have less resistance. Theyre not as good at keeping dirt out as rubber shielded but inside a transmission...