One of them is incorrect for the diff then. That's just the way they were built. You can see it in the eBay picture and in the illustrations. One is longer than the other. That's why you get the same result when you swap them. Because yours are the same length when they shouldn't be. If you bought the diff used it might be that the seller popped the axles out for storage and gave you a mismatched set by accident. I used to have several R200's and I stored them with the axles removed sometimes. Anyway, in short...your stub axles should be of two lengths, not one. Good luck.
I'm a big fan of having a vacuum gauge on the balance tube (and plugging the emmissions holes). If the engine isn't healthy, it doesn't matter how the carbs are tuned. Compression, Timing, then Carbs. The vaccum gauge is a wonderful tool to see the health of the engine. I