8-3/4 rear axle bearings do not use the gear lube for lubrication that's why they have an axle housing inner seal --- 8-1/4 -- 9-1/4 and similar rear axles do use gear lube to lubricate the outer axle bearings
if you have an 8-3/4 with the tapered axle bearings use a good chassis/wheel bearing grease to pack the bearing -- same grease used for the front wheel bearings