So, I'm getting ready to put the rear end back together this weekend and I am looking at the rear half-shafts..  I marked them before I took them out, but I'll be darned if I can see any differences.  I took some measurements and they seem to be identical in all of the critical dimensions. 

Both shafts are exactly the same, so they will fit on either side. The main reason to mark them when removing is to keep the balance correct...assuming they have never been removed before. Once they've been off, and not marked, or replaced in the exact same position, it don't really matter anymore. Just changing the position of one end can help with a vibration problem, if you wind up with one.