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It seems like the gears were incorrectly installed, and the end result is...well...you see it yourself. You are most likely going to need a new diff, as even your case is cracked.

 

Hard to say of thay case crack is just casting lines/marks or indeed a crack.

I would agree on the bad gear install - how long have the newer gears from Ricks been installed for?

 

I had the same thing happen to my 4.57s and Comptech reinforced housing.

My gears were installed by Hardtopguy.

Lasted about 17k miles.

I'm happier with my current stock gearing. Better for autocross... less shifting. I keep a spare OEM diff in my garage for when this one goes boom.

 

I just don't understand how this can happen in a sudden manner right after the fluid change.
I'm taking down the diff tomorrow for the further inspection, but just don't feel comfortable enough to open up the casing.
the new gears is about 16k old

I'm really thinking about going back to stock gearing.
what was the cause of your damage CKit?