I have a 1990 Range Rover. When the A/C is on and I accerlerate, there is a loud screeching noise. Also, sometimes there is a ticking at my feet and when I press the brakes it is firm and slips. Sometimes the steering gets stuck and shakes violently, around this time there is a clunk in the undercarriage as if the gears are changing. Any help or advice would be much appreciated.

 

Your are most likely talking about many problems.
For the A/C related noise, I would suggest you inspect for a loose or worn belt and or idler/tensioner pulleys failing. That would also cause the steering problem cause your belt is slipping.
The klunk could be anything, drive shafts, u-joint, slip shafts loose or warn steering componets, the list goes on. You will need to have someone check that out.
Give a better description of what your brake noise is, what it feels like, do you loose your brakes, does the pedal go to the floor???
Mike