I think that I have developed a small crack on the fill hole for the rear differential. I believe this is the cause of my leaking fluid. So, first off could fluid make it through a crack that is above the fluid full level? And, also, is there any way to either seal this, or replace just half of the differential case? After being quoted $700 for a new rear end, I hope there is some alternative. Thanks.

 

are you sure its cracked, cus mine has small lines that look like cracks, that are from the mold it was pressed in.

 

Yes, fluid WILL leak out of a crack above the full level.

 

If you clean it really good, with scotchbrite pads, and wipe it clean with acetone, you could apply some JB Weld.

 

Yep, a good epoxy might seal the crack, but WHY is there a crack??? If the crack is from stress, NOT a factory defect, then it's a good bet the crack will grow, ruining the epoxy seal. Make a little scratch mark where you think the crack ends, then ride for a while. If the crack is growing, no epoxy is going to help.
I'd say remove your diff, dissasemble to the case, and have it welded for $25. Then, your problem will be solved.

 

Do you think it was caused by overtightening? I'd say that welding is the only way to go.

 

My eiger has a crack in the same place! It cracked when I was tightening the plug, I think it is a weak spot in the design because I was using a small Allen key and wasn't torqueing the hell out of it. Oil in the threads can build hydraulic pressure add a weak design and you have why it happened. I've used a liquid seal but it is only a temporary fix, I'm going to either JB Weld it or tear it apart and get it welded.