I haven't looked into it, but it's my assumption that a bearing grease (vs oil) is engineered to handle the high pressures of a loaded low speed bearing such as the rears on an FF, where gear oil being thinner doesn't provide that level of protection for high metal to metal pressure. I would be interested in hearing whether this is in fact true. After all, I'd think that the gear oil is handling some pretty high pressures with the diff gears, etc. Dunno.
I know of at least one other 80-ish application where Toyota chooses grease and another mfr switched to gear oil. Land Rover (I know, I know) switched to gear oil in their birfields at some point years ago. Wouldn't it be nice to change out your birfs by removing a drain plug and filling it like a differential? Of course, there are some down sides to the LR approach, but just another data point.
As Robbie points out, my rear bearings did get a bit loose and put some machined marks on the rear spindle/axle housing. This was on the 93, which was used for some heavy towing with just gear oil (washed all grease out) in the rear bearings for a couple years. I didn't seem to have any trouble getting the bearings to set up correctly when I repacked them, but as mentioned above curiosity has now taken hold and Cdan's sending me a rear seal kit so I can dig in and have a look see. Will post results.
OTOH, that machining damage happened by 90,000 miles yet my 97 had 137,000 with oil - washed bearings and not a trace of a mark. So, some evidence to attribute that damage to the presence of gear oil (vs specc'd grease) when the vehicle's being used heavily on the rear axle. My 97 was just a mall cruiser, and rear bearings awash in gear oil may be just fine based on my findings, but I wouldn't tow with it like that.
DougM
DougM