Well from your mechanic's actual statement then he's wrong. If it's a full-floater then the gearlube from the rear diff does NOT get to the bearings.
Now if the full-floater part was wrong, so you really have a semi-floater then yes the gearlube from the rear diff lubes them.
If you have a full-floater then there are two wheel bearings in there, exactly like the front.
The easiest way to tell if you have a full floater is if the rear axle sticks out inside the rim. A full floater will have a round part (the hub) that sticks out and you could set the wheel on it while you are aligning lugnuts. The center part sticks out about 4-5". A semi-floater does not have that, so for lining up the lugnuts there is nothing to set the rim on...I think those observations only stand true for the Cruiser though, Toyota trucks are a different setup I think.
Hope that makes sense...
My '81 FJ40 is semi-floater (C-clips hold the axles on)
My '97 is full floater...