Hi Sir or Madam, I am John Mc, a Ford dealer technician.I hope I can help solve your problem quickly and accurately.
Is it the rear diff that is bad or the transfer case? Also what is the symptom of the problem so I can better estimate for you?
This is usually caused by binding differential clutches in the REAR differential. This is a very common problem. To fix first just try changing the gear oil and add two bottles of Ford friction modifier. When done go to a parking lot and drive in 10 tight circles to get the clutches working correctly. This is around a $100 to $150 repair. If this doesn't fix it the rear end clutches will need to be replaced. To do this it is around a $600 repair because the diff has to be removed.
If you have any more questions please feel free to ask, it's what I'm here for!
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