I'm looking in a Parts Catalog and it appears the center shaft is held in place by a locking pin in one of the small gears at either end of this shaft.

I just had to do differential work on a GM product, and the center shaft was held in by a locking pin that was screwed in like a bolt. This one looks like it might be pressed in somehow. So watch the gears for a locking pin as you turn the assembly around.

I can't do the "wheel back and forth" test right now on my car because its up on ramps for a starter replacement (hopefully this weekend. )

The key to differential disassembly appears to be getting that little center shaft out.

Ken300D