Jeep ZJ Power Steering Pump Problems

Hi, I have this problem and need some help diagnosing whats wrong. Yesterday I was doing some doughnuts in some dirt/ light mud. In the middle of a doughnut, my 1994 Jeep Grand Cherokee (6 cyl. 4.0L) lost power steering. Now, especially when at idle, or when parking, it is really hard to turn the steering wheel. There is also a low pitched whine when I turn the wheel. When I turn the wheel all the way, hitting the bump-stops, I hear a loud "vibrating" noise- it kinda sounds like the fan hitting something(but it isnt). When this happens, the power steering pump and hose that comes out of the top of the pump/ reservoir vibrate pretty fiercely. Another thing that I noticed is that when I am going very slow (5 mph +/-), like when i'm parking, I can shift int neutral and rev the engine up a little and that makes it a LOT easier to turn the wheel. That is why I think the problem may be in the power steering pump. I have the correct level of P/S fluid, so that isn't the problem. Although the fluid is a dark brown, indicating that it is dirty. Could this be the problem?
IF ANYONE COULD HELP ME DIAGNOSE THIS PROBLEM, PLEASE POST A REPLY OR E-MAIL ME AT Snofellow2003@yahoo.com. Any help would be greatly appreciated.
-Chris