So I just finished replacing my rear axle seals and changing the fluid, I get it put all back together and fill up the differential and it turns out the fill plug hole is stripped, the plug just never tightened up, it looks like the previous owner just glued it in. what is my best permanent fix for this that's not going to get aluminum shaving in my diff.

 

I think a helicoil would be the best repair for you. You would have to find you thread pitch & diameter & look around at auto part stores for the size you need.
If you want to do a temporary fix i would clean all the old silicone off , wipe clean the threaded area with brake clean & a rag & re silicone the plug back in. Let it dry overnite & you should be good.

 

In order to do a helicoil you'll have to drill and tap the diff housing to a larger size, which will introduce shavings into the diff. Even so, this is probably the most permanent solution other than pulling a used diff out of the junkyard.

There really shouldn't be any strain on the plug, so RTV should be fine to hold the plug in. For peace of mind, I don't think I would run it this way for very long.

 

I found some permatex thread fixing compound at the parts store so i'm going to give that a shot and see. I wish i could just go to the junk yard but none of them around here have Audi's of any kind.

 

and if you do thread the diff, use petrolium jelly to help with the shavings

 

What Jeff and husted said. Normally I'd suggest JB but it's a fill plug so that's a bad idea. If it's the threads on the plug itself, you can try re-cutting the threads and see if that helps.