1992 BMW 318i rear differential

I just bought a 1992 bmw 318I. Was told it needed a rear diff. Which a friend got for me used. When I got under the car I saw that the front mounting bolt was missing. But there was a piece of the bolt still in the diff. So either way it had to come out to extract the bolt. So I pulled it out. When I compared the new to old diff I noticed that (on the new one) if I hold the mount still where the driveshaft connects to it, I can still spin the axles (both sides). Now I'm no expert and have never done this before, but that's not right. Right?
All moving parts on the diff should move in sync with eachother right?
The original diff doesn't do that.
Which one should I use. The original seems tight and spins all together.
Thanks guys hope to here from u soon