90 disc brakes on rear

Hi
This must come up often
in order to get rear disc brakes on my 1987 90 (currently drums)
which axle is compatible, heard its a discovery 1 rear axle, is this correct, and is the gearing ratio the same as in my diff?
years ago I had a 110 and put a kit on that one but expensive
if it is a disco axle has anyone got one for sale (cheap!) out there
not in a rush but would like to investigate.

Hope someone can help

thanks
Dave