if you are getting any differential oil leakage from around the outside of the hub assy you probably have an axle seal bad. If that seal goes it allows fluid to enter the jack shaft and bullgear area that can fill that area up with oil and if the bullgear cover and end housing gaskets are not sealed correctly the oil will seep out.
if the gears are dry then what kind of grease is being used inside the bullgear area? It should be a EP type grease.

As far as the speed of the truck travel, you might want to check the drive pressures, linkage adjustments etc.
Is it slipping any while traveling? Try stalling the truck and check for that.