Locked Rear

What would cause the rear of a 2002 Ford Explorer to lock and also cause it to leak oil and transmission fluid? The vehicle has approximately 207,000 miles on it. The truck starts and when put in drive it will rock as if attempting to move but won't. I hear a slight clinking sound from the rear of the truck. I recently started leaking transmission fluid whenever the gas pedal is pressed. There is also oil on the body of the vehicle underneath the rear. Any ideas what the problem is?