when i put the gearbox into low range i cant use reverse, does this mean i need a new gearbox or a transfer box?

Ive recently bought my first defender and so far so good but... Ive tried taking off the tree sliders as they are all rusty and need a refurb. I have taken off the nut at bolt at the front and the slider comes loose but i cant seem to free the rear of it. Am i wrong in thinking there is no other...