4kq center diff failure

[TM]

Hi listers,
I wanted to get some feedback on this issue- a friend of mine has an
'85 4kq with approx. 90k miles on it that is suffering from imminent
center differential failure, as diagnosed by my mechanic. The center
diff
makes a horrible noise according to my friend, that he describes as
sounding
like the transmission is about to explode, but only occurs at cold
temperatures. He has changed out the trans fluid to no effect.

Thinking (as does my mechanic) that this is highly unusual, any
suggestions
on what the cause may have been for the diff to start to destruct?

Due to this, he's looking for a good 4kq transmission w/ center diff.
Any
leads would be appreciated.

Taka