Differential replacement/repair question

[The Quinn's]

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Hello,
I have been off the list for about a year , since we sold our '95 90q
I recently test drove an '88 non-quattro 90 (automatic), which was very well maintained, but has a differential problem.
I have searched the archives, but was not able to get a clear picture as to what is involved with a differential repair (how much will it cost/how long would it take for me to do it?).
Thank you,
Pat
Kilauea, Kaua'i
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