depending on how long you intend to let your car sit before being driven will determine what you use.
if you are driving it right away after assembly then use gear oil as pre-lube. if it is going to sit for a while i would use bearing grease. reason being is that the diff fluid sits in the sump and doesnt travel down the tubes to the outer bearings until after your down the road aways. just helps lube the bearing until gear oil from the sump gets to it.