Oil Leak at the Rear End

My 1991 Toyota Hiace campervan is leaking oil at the rear end (not up the front under the engine). The axle that joins the rear wheels meets in the middle at a "hub" - not sure what it's proper name is. The oil is dripping from the hub. What is the name of that part of the van ? I realise I need a mechanic to take a look at it but wanted to be armed with a bit of info first ! I'm not meachanically minded so won't be attempting repairs myself.

Thanks
Kelly