I opened up the differential because of a bit of bearing noise I thought, but found the magnet totally covered in metal shards and the tip of a gear-tooth. I cleaned everything up, then I noticed that one of the two planetary gears is basically shredded. I'm always at 75 80 miles an hour on The Big Road but I don't hot rod around town. Needing to work, all I could do was button it up and put fresh oil in her.
is it anything major to change that one gear or if I can find a good used set? Any good ideas on where to find gears at? Oh, and I'm at 218,000 miles.

 

I get the impression that at this point it's going to need a rebuild. Might be cheaper to swap the whole axle.

 

Tearing it down to replace one spider gear would be the same as tearing it down for a complete rebuild/gear replacement.

Given the mileage, I’d recommend just replacing the carrier (ring gear can be transferred to new unit) especially if time is a factor. For one gear to be shredded means there’s been excessive heat and low fluid level at some point and that can cause fatigue in the carrier. You might do the rebuild just to have the carrier crack/break after the first few hundred miles and then you’ll be replacing it anyway.

Might as well replace the axle shaft seals (and bearings if questionable) while you’re at it. Thoroughly clean the tubes (a piece of steel dowel with a hook on one end and shop towels work well), housing and vent to make sure all the metal shavings are cleaned out. If it’s a limited slip don’t forget to add a friction modifier to the fluid.