Found this at atlantic british's web site while trying to research potential disco problems. This was the first thing on the list so it is probably real common. hope it helps.
http://www.atlanticbritish.com/techtips_main.html#discovery
Can't shift out of park - Troubleshooting
Vehicle - Land Rover Discovery and 1995 Range Rover Classic
Check to see if you have stoplights. No lights check:
a. Fuse f1 in the fascia fuse box ok? (Discovery, if the fuse the blows check the terminals on the back window grids making sure they are not touching each other causing a short. They get bent easily)
b. Inspect the brake light switch on the brake pedal (The brake light switch has four wires on it and the cruise switch has two wires and a vacuum line on it).
If switch is intact:
a. Jump the green/purple wire and the green/orange wire; the brake lights should come on enabling the shift out of park.
b. Caution! The brake lights will be on all the time. Have problem repaired as soon as possible. Also you may get an ABS fault stored.
If you have stoplights pop the top of the shift lever off and check to see if the clip on the button is intact. They have been known to come off and not lift the rod in the handle.
1992-1994 Range Rover Classic:
Same as above except: Dash fuse box fuse# B1 and jump wires: white/purple and green/purple at brake light switch #AMR2010G.