Get with shaun from all american street car performance. He is on here as na306stang or john username tiffiny. I am more then sure they are beyond adequate to take care of your issues whether you change the gears and pinion bearing or replacing the whole axle.
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Ryan
1990 Mustang GT (In Process)
1993 LX Coupe (Daily Driver, but just choose not to)
2005 Excursion (Big Pig)