Gated ones have to be moved sideways to get into 4lo. Non gated go in a straight line.
A little oil in the intermediate housing is not too bad. Just means the seals were leaking a little. Can you tell if it is from the t-case or transmission? Either way I would put a new seal on the t-case input. Unless you turn the coupler down you will have to remove the seal on the back of the NV4500. I used a double seal on the input of my t-case to hopefully prevent contamination. So far so good.
Unless you really plan on abusing the truck, you won't need the cast iron housing.